conversation: re-design sending/sent status

The goal of this patch is to allow the clients to get a better
sent/read status from the daemon.

API doesn't change much, but internal logic got some changes. For the
client:
+ SwarmMessage now contains a map<string,int> status where string is the
  uri of a member, int is the status (0 = sending, 1 = sent, 2 = read)
+ cancelMessage is removed as not used anymore (sendMessage with flag=1
  will edit a message)
+ getMessageStatus is removed as the status is sent in the SwarmMessage
+ accountMessageStatusChanged is now emitted for swarm messages when a
  fetch or setDisplay occurs. Client must handle this signal correctly.
+ Previous code to manage last displayed, fetched status is now merged
  with message status
+ Sync info is done when the sync is opened, else status are not updated
  correctly

GitLab: #948
Change-Id: I60763d4de8a995c6fc9f6df6434e266211f8dc2f