conversations: prevents received messages from reverting status

- Message engine will cause erroneous status updates
  to SENDING or FAILURE for messages already SENT or READ. This
  patch treats these cases client side.

Change-Id: Icd5df2728bb83f8a4ba193f653e12f68935aa4bd
Reviewed-by: Kateryna Kostiuk <kateryna.kostiuk@savoirfairelinux.com>