Add deposit info to objects added to swh-storage from metadata-only deposits

Deposits with code objects are loaded as their own origin, so we can
look them up in the deposit database from their metadata (which hold the
origin as a context).

This is not true for metadata-only deposits, because we don't create an
origin for them; so we need to store this information somewhere.
The naive solution would be to insert them in the Atom entry provided by
the client, but it means altering a document before we archive it, which
is bad.

This commit makes the deposit server write a "metametadata" object (ie.
a metadata object with an other metadata object as target) the metadata
storage.
This metametadata contains information on the deposit itself: id,
client, and collection.