Inconsistency between DAO and MD-SAL config datastore at startup

After a controller restart, with a non-empty set of registered mappings, the mdsal config datastore is correctly restored, however the dao is empty. Apart from the inconsistency, this also prevents manually re-registering the mappings to dao via either rpc/restconf calls. That is, because the mappings are already in the config datastore, overwritting them with identical mappings will not generate notifications needed for replicating these mappings to dao.