DB schema: merge directory_list_* tables into directory

It is kinda pointless to have 3 tables (directory_list_{file,dir,rev}) that only add one field each to the directory table. As we are using arrays anyhow, they could as well be fields in the directory table itself.

Work in progress to fix this is in the branch zack/dir-lists-in-directory of swh-storage. It includes schema changes and SQL upgrade script, but not yet the porting of existing stored procedures. This work should be merged with care, as it impacts the git loader.

Also: size impact hasn't yet been quantitatively evaluated yet.

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Migrated from T54 (view on Phabricator)