CassandraRecipientRewriteTable: listSources support

In `CassandraRecipientTable`, use the `CassandraSchemaVersionDAO` to determine what to do:

• if schema version is 6 - or less: we know that entries might not be migrated yet in the new DAO. We can not rely on it hence we back up to the API default method - safe but slow.
• if schema version is 7 - or more: we know that all entries are present in the new DAO, then we can use the new DAO - faster