Close T709

This allows loaders to lazily load data: you can read from disk and transfer
only the contents that are really missing.

Summary:
This module is a reimplementation of swh.model.git, with the underlying goal of
replacing it and fixing T709 in the process.

Reviewers: #reviewers!

Differential Revision: https://forge.softwareheritage.org/D248

Currently just a stub (but with a hereby fixed anchor) to allow linking to it
from other part of the development documentation. Will be fillen later on with
an abstract description of the Software Heritage data model.

change cherry picked from python module template
commit 71b117ba0cf9f1251b1cac26d0994df03a4c787d

Summary: for those people with legacy openssls

Reviewers: anlambert, #reviewers!

Differential Revision: https://forge.softwareheritage.org/D224

Fix T685 out of spite.

While we wait for a cleaner refactoring of this code, this fixes the immediate
clogging of the database with bogus data issue.

Related T703

This resolves the caveat mentioned in prior commit about not being
able to use blake2 prior to 3.5

Related T692
Closes D192

- Add module docstring
- Add blake2s256 and blake2b512 in supported algorithms
- Spawn a new variable DEFAULT_ALGORITHMS as default computed
  algorithms for the main functions

Related T692

Remove the limit on the python3 version, this should be transparent.
If the hash requested is not available, this will raise with an
explanation on the error.

Related T692

The same caveat applies, will only be supported from python3.6 onward.

Related T692

The caveat is that it will only be supported when we will be using
python3 >= 3.5.

Related T692

Related T700

Closes T255
Ref. D179

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.