conversationmodel: fix deadlock on first interaction

af04c34f introduced a condition variable used to block until the
conversation was created, circumventing a race condition when
attempting to insert interactions. However, when not using D-Bus,
slotContactAdded will be invoked on the blocked thread, and thus
deadlock until the wait_for timeout.

This patch replaces the synchronization mechanism with callbacks.

Change-Id: I5f975a6245232e586be7df3f29008c1162d6b208
Reviewed-by: Sebastien Blin <sebastien.blin@savoirfairelinux.com>