Support removal of ExtID and RawExtrinsicMetadata objects

These changes are needed to allow swh alter remove to remove RawExtrinsicMetadata and ExtID objects from the PostgreSQL and Cassandra implementation of the storage.

As RawExtrinsicMetadata objects can be addressed using an ExtendendSWHID, they can now be removed using object_delete().

For ExtID, a new method extid_delete_for_target() has been added to ObjectDeletionInterface. It will remove any ExtID object targeting one of the object in the a list of SWHID given as an argument.

Note: also contain a bugfix for transactions with object_delete()

Address swh-alter!11 (merged)