Implement the initial MonogDB backend with a simple data model

Data model used

content                                                                                                                                                                                                            
{                                                                                                                                                                                                                  
    id: sha1                                                                                                                                                                                                       
    ts: int //optional                                                                                                                                                                                             
    revision: {<ref revision str>: [<ref path>]}                                                                                                                                                                   
    directory: {<ref directory str>: [<ref path>]}                                                                                                                                                                 
}                                                                                                                                                                                                                  
                                                                                                                                                                                                                   
directory                                                                                                                                                                                                          
{                                                                                                                                                                                                                  
    id: sha1                                                                                                                                                                                                       
    ts: int  //optional                                                                                                                                                                                            
    revision: {<ref revision str>: [<ref path>]}                                                                                                                                                                   
}                                                                                                                                                                                                                  
                                                                                                                                                                                                                   
revision                                                                                                                                                                                                           
{                                                                                                                                                                                                                  
    id: sha1                                                                                                                                                                                                       
    ts: int // optional                                                                                                                                                                                            
    preferred  <ref origin>  //optinal                                                                                                                                                                             
    origin  [<ref origin>]                                                                                                                                                                                         
    revision [<ref revisions>]                                                                                                                                                                                     
}                                                                                                                                                                                                                  
                                                                                                                                                                                                                   
origin                                                                                                                                                                                                             
{                                                                                                                                                                                                                  
    id: sha1                                                                                                                                                                                                       
    url: str                                                                                                                                                                                                       
}                                                                                                                                                                                                                  
                                                                                                                                                                                                                   
path                                                                                                                                                                                                               
{                                                                                                                                                                                                                  
    path: str                                                                                                                                                                                                      
}                                                                                                                                                                                                                  

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Migrated from T3561 (view on Phabricator)