Related T494

This will allow us to handle another storage backend for the storage of the
archiver data.

By default we would try to copy objects from all the archives, even those for
which we didn't have a configuration.

The default value for content copies is "missing", so we don't need to make it
explicit.

Implementation wise, this uses COPY statement and drop duplicates if
encountered for content_archive insertion.

Use the same insertion pattern as swh.storage.content_add.

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

Related: T494

This use case cannot happen with ArchiverWithRetentionPolicyDirector:
- If a row entry is referenced in the archiver db, it's present in
  the objstorage

- And if a row entry is not referenced in the archiver db, it
  won't be listed as missing since it's the archiver db which is
  read for listing the contents we want to archive.

Related T494
Related T569

Related T494

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.

Close T672

The buckets are using the last two bytes of the object id, so that we spread the
load across different lines on sequential archivings.

Closes T646

The check_config method allows a dynamic check of the configuration for
a running storage. We can make sure that we have proper permissions on
the object storage as well as the database before running things.

Related T575

The 'unknown sha1 path' cannot happen in the default archiver since it
reads from the archive db (so the fallback code is not necessary in the
worker).

To the contrary, since 'archiver to backend' reads from stdin (for now),
we could have unregistered sha1s from that source.

This commit makes the director deal with that before sending sha1 to
workers.

It's also the director's job to set the state to 'missing' when the
force_copy is true before sending sha1 to worker.