This commit changes the swh pids representation returned by the
parse_persistent_identifier function from a dict to a named tuple.
Also, the 'object_type' attribute of that named tuple now contains
a full name instead of an abbreviated one.

Closes T1112
Closes T1125

Close T1104
Close D346

Related T1104

As the client of this api might not be aware of what's the right id
key for a given object.

Related T1104

Related T1104

Related T1104

Related T1104

This commits adds support for parsing optional parts in persistent
identifiers. Optional parts are defined the following way:

	<swh_id>[;key1=val1][;key2=val2]

Related T1040

Related T335

Related T335
Related T933

Summary:
Add support for snapshot identifiers

Close T566.
Related to D268.

Test Plan: Unit tests included

Reviewers: zack, #reviewers!

Maniphest Tasks: T566

Differential Revision: https://forge.softwareheritage.org/D277

Related T703

Found by investigating T75

The subversion loader (T680) has shown that throwing floating point values
around for timestamps is a mess waiting to happen.

We now coerce all clients to send us timestamps as integer numbers of seconds
and microseconds, avoiding data losses everywhere.

Move timestamp normalization to another function to make it more easily
movable.

This function wasn't in sync with what's supposed to be our revision
schema

so no bytes, and we enforce during the checksum computation function

T270

First step towards T68.