DCB: Monitor should return poll status based on last retrieval

Currently DCB Monitor returns PollStatus.unknown when it is not scheduled to retrieve config. Instead, we should return poll status based on last retrieval for the device ( that may have happened with manual trigger)

Usecase :

Retrieve the config from Monitor failed and then after some config changes, manual trigger succeeded to retrieve the config. But Monitor still have the same failed status.
Instead of we should update the poll status based on last retrieval ( that may have happened with manual trigger).

Also fix multiple updates for the same device ( synchronization in persistence).
and Exception when retrieving non-existing interfaces