Unsets should always be considered successful

[rbino]

This is related to #438, which fixed this for properties setting but didn't fix it for properties unset. For the same reason, unsets should always return 200 OK regardless of the number of matches on the broker because:

• If a session is present, the unset will be sent when the device reconnects
• If a session is not present (or the device asks for a clean one on reconnect), the information of the unset path will be delivered by the /control/consumerProperties message.

Incidentally, there's currently another bug because the unset gets saved in the database before checking the return value of ensure_unset, but this was not put in a separate issue because after this issue is fixed ensure_unset will always succeed by construction, thus making it always valid to save the value to the DB before it is sent.

Reported by @szakhlypa

[Annopaolo]

Closed by #691.