conversation: fix message status computation

This patch is a complete rewrite of the algorithm used in the
addToHistory function to compute the status (SENDING, SENT or
DISPLAYED) of the added messages. This fixes a bug where some messages
would sometimes be incorrectly marked as being in the SENDING state,
even though more recent messages were known to have been SENT or
DISPLAYED.

GitLab: #1042
Change-Id: Ibfe0aeee57523348a5f8430492814572e607090e