conversation: emit signal when conversation cloned

Currently, when a contact accepts an invitation while
iOS is in the background, the extension stops the
daemon before synchronization is completed. This
patch allows the client-ios to know when the fetch
is completed, so it can wait until the conversation
is cloned.

GitLab: #776
Change-Id: I123f3ca94c1cece1138db09d536f48e6533cf78f