message engine: don't persist sent/failed messages

* Avoid message accumulation by not persisting the state
of failed and sent messages, which are already saved by clients.
Clients are still able to know the state of a sent/failed
message sent during the same session, and are expected to catch
and persist the message state signal to know when a message failed
or was successfully sent across daemon restarts.

* Save pending messages at every state change and don't
delete the previous file to avoid loosing the message state
in case of crash.

* Make message saving asynchronous to avoid a slow/high latency
storage device to block the UI or DHT at every message

Change-Id: I96221152a86990c8e9f1be13903a675a87e4e975
Reviewed-by: Andreas Traczyk <andreas.traczyk@savoirfairelinux.com>