Implement storage of the ExtID.extid_version field

This fields allows having multiple version of the ExtID -> SWHID
mapping, for instance when the implementation of a loader changes in a
backwards-incompatible way.

For now, we don't change the API used to query or store ExtIDs. When
querying for the SWHIDs corresponding to a given external objects, all
versions are returned, and the client is expected to do the filtering.