The current postgresql model refuses NULL values.

Related to P589

Related to T2188

Add support to automatically compute identifier in the following object models:
Directory, Release, Revision, Snapshot.

If the identifier is not provided as parameter, it will be computed when the model
is initialized.

we do not really need them to be mutable, plus we gain their instances now
being hashable, so we can add them in set() for example.

Instead of relying on attr.asdict recursion, we do recursion ourselves. This
simplifies a lot of the inherited to_dict() methods.

Needed by the replayer in swh-journal.

It kind of matches the current state of the postgresql storage, which
does not support it.

But keep support for deserializing from str, like
swh-storage does.

Code manipulating a Content object may want to access a hash
of configurable name; this method allows it to do that
without using getattr directly.

It denotes files whose length is unknown.

Loaders use it, but it is ignored by the pg storage.
However, as the Cassandra storage uses swh-model to validate
its input, it refuses this input from the loaders (and journal).

There is more repetition, but it's easier to read and
'%timeit Revision.from_dict(d)' is 5 times faster.