Add experimental option to reduce extremities.

Adds new config option `cleanup_extremities_with_dummy_events` which
periodically sends dummy events to rooms with more than 10 extremities.

THIS IS REALLY EXPERIMENTAL.

synapse/federation/sender/__init__.py

@@ -168,6 +168,9 @@ class FederationSender(object):
                     if not is_mine and send_on_behalf_of is None:
                         return
 
+                    if not event.internal_metadata.should_proactively_send():
+                        return
+
                     try:
                         # Get the state from before the event.
                         # We need to make sure that this is the state from before