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.