related to T1242

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

Closes T1177

Related T1061

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

Workaround while the scheduler doesn't handle priority tasks

Summary: This should make tests more repeatable across different environments.

Test Plan: Run tests with a non-empty .gitconfig, for instance having commit signing turned on

Reviewers: ardumont

Differential Revision: https://forge.softwareheritage.org/D310

Summary:
When running the tests, we expect the local MTA to be working. That's not something that we can assume.

The notification tests all mock the SMTP object to check whether it's called
properly, so we don't really need to connect on backend initialization.

Test Plan:
Run tests with local MTA disabled, make sure they still pass.

Group Reviewers: Reviewers

Reviewers: ardumont

Reviewed By: ardumont

Differential Revision: https://forge.softwareheritage.org/D308

Closes T937