Related T1532

Prior to this commit, during the case of committing in another
branch (ref), there were dangling files in the HEAD branch.

pifpaf is not needed any more.

- Ensure every service for which an (remote or local) access is needed uses the
  same config structure (dict with 'cls' and 'args' keys).
- Extract instanciation (thus confguration) of remote/external services
  (storage, cache, scheduler) from the VaultBackend constructor; these are now
  passed as __init__ arguments; their instanciation/configuration is thus now
  handled by swh.vault.api.server and swh.vault.cooker.get_cooker (with some
  support in swh.vault.get_vault).

so that we have a chance to explicitely choose the config file used when
running the wsgi app using a wsgi server like gunicorn.

Typically used by external wsgi server like uwsgi or gunicorn, e.g.

  gunicorn swh.vault.api.wsgi:app

required to pull storage's test dependencies (like hypothesis).

related to T1330

Summary: Reported by Ludovic Courtès on IRC

Test Plan: make docs

Reviewers: seirl

Differential Revision: https://forge.softwareheritage.org/D674

We need the postgres dependency to run the tests

Related T1247

this require a small change in vault's swh-schema.sql: create the dbversion
tqble only if needed, otherwise this sql cannot be executed without
error on the top of storages' db schema. I guess it used to work thanks to
the fact theses used to be pg_restored instead of SQL statements executed via psql (with '-v ON_ERROR_STOP=1', which se do want).

Related to T1265.

Related to T1265.

related to T1242

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

Closes T1177