To avoid storage server timeouts when recursively iterating on a large
file tree, perform that operation client-side.

Closes T1177

Related T1061

Summary:
Also make sure storage objects get garbage collected properly, so that automatic
database cleanups keep working.

Test Plan: make test passes when D324 is applied to swh.storage

Differential Revision: https://forge.softwareheritage.org/D327

Related T700

This also has the benefit to hide some celery name (which is an
implementation detail from swh.scheduler).

To make sure corruptions such as T680 don't happen again, use the same
normalization function as swh.model before inserting timestamps into our
database.

This makes swh.storage reject non-integer timestamp values as well.

Update tests to reflect this change.