Store arbitrary relations from events. (#11391)

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.

tests/unittest.py

@@ -331,7 +331,12 @@ class HomeserverTestCase(TestCase):
             time.sleep(0.01)
 
     def wait_for_background_updates(self) -> None:
-        """Block until all background database updates have completed."""
+        """
+        Block until all background database updates have completed.
+
+        Note that callers must ensure that's a store property created on the
+        testcase.
+        """
         while not self.get_success(
             self.store.db_pool.updates.has_completed_background_updates()
         ):