conversation: store last displayed messages

With this patch, the daemon is now storing informations based
on setMessageDisplayed/onMessageDisplayed to follow the read
status per participant.

This avoid to creates a logic in all clients and allow the client
to have a correct read status even if the daemon was launched
separately.
Clients can now:

+ Retrieve last displayed interactions by retrieving members infos
(via getConversationMembers() and the key "lastDisplayed")
+ Know how many messages there between the head and a commit. This
is useful to count unread messages for a peer.

GitLab: #592
Change-Id: I0b5bc7adb5cda48754f33a362a40ce10ee345e3c