conversation: pass linearizedParent to client

loadMessages didn't return any linearizedParent because we were
passing a value to the client before the update.
Also, last message loaded will not have a linearized parent (because the
parent will not be loaded yet), so we need a message updated signal
at the next load.

Change-Id: I01c46f2c34be2131abd731eb745c203e50b95b9a