Instead of only known relation types. This also reworks the background
update for thread relations to crawl events and search for any relation
type, not just threaded relations.
Adds validation to the Client-Server API to ensure that
the potential thread head does not relate to another event
already. This results in not allowing a thread to "fork" into
other threads.
If the target event is unknown for some reason (maybe it isn't
visible to your homeserver), but is the target of other events
it is assumed that the thread can be created from it. Otherwise,
it is rejected as an unknown event.
