Closes T1845

Related T1845

Related T1913

Finally manage to remove the ugly hack I introduced in a2759fec. Turns out
that calls to file-magic API are not thread-safe so they must be protected
with a threading.Lock instance.

swh-web uses the magic module to detect mime_type and encoding of contents stored
inside the archive. Its is provided by the python-magic package (debian buster)
or by the file-magic package (debian stretch).

As python-magic is more stable than file-magic (see a2759fec), prefer to use
it instead. Nevertheless, keep backward compatibility for file-magic as it is
currently used in production environment.

The vault backend sends an email containing an invalid url to fetch gitfast archive.
So setup a redirection to the correct one as a temporary workaround.

In addition to the 'normal' tests, which test against the
current behavior of the pg storage.

Related T1909

It's no longer used.

The former only supports an origin id, and fetches a whole snapshot
while we're only interested in knowing if there is such a
snapshot here.

Related T1900

This function will get called a lot (by default, up to 70 times
per origin search), so it deserves a much higher rate-limit.

It will replace the 'internal' endpoint _origin_latest_snapshot,
and won't require fetching the whole snapshot just to know
if there is a visit with a snapshot.

It will be removed in the next release of swh-storage (v0.0.146).