conversation: move logic of message management in daemon

This heavily changes the API for the client. The goal here is
to move the logic to construct the history to show in the daemon
and not the client. This has several advantages:

1. Logic is common across every platforms, so bugs should not be
platform-specific
2. Client got less logic
3. Signal are simplified, if an edition comes, "MessageUpdated"
will be triggered instead MessageReceived.
4. Some tests are added for linearizing the history.
5. Search on edition is fixed.

Tests got heavily re-written, but the content didn't change (2 tests
are added, the rest is simplification).

GitLab: #831
Change-Id: Ie7c81077067e9e49db1dd396829c9225c0512c16