Always return the own message when asking for a thread

The front-end asks for the thread of a message by sending the message
ID. Now, after an upgrade from v1.4.x to v1.5 and no sync of the
mailbox, the `thread_root_id` will still be `NULL`, hence we can't find
related messages this way. But we don't have to look it up that way, we
can also allow matching by the ID.

This safes me from coming up with a complicated strategy to force a
resync after the upgrade. Messages will be able to show. The threads
will build deferred.

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>