Merge pull request #6837 from matrix-org/rav/federation_async

* commit 'c7d6d5c69': (27 commits)
  make FederationHandler.send_invite async
  make FederationHandler.on_get_missing_events async
  changelog
  make FederationHandler.user_joined_room async
  make FederationHandler._clean_room_for_join async
  make FederationHandler._notify_persisted_event async
  make FederationHandler.persist_events_and_notify async
  make FederationHandler._make_and_verify_event async
  make FederationHandler.do_remotely_reject_invite async
  make FederationHandler._check_for_soft_fail async
  make FederationHandler._persist_auth_tree async
  make FederationHandler.do_invite_join async
  make FederationHandler.on_event_auth async
  make FederationHandler.on_exchange_third_party_invite_request async
  make FederationHandler.construct_auth_difference async
  make FederationHandler._update_context_for_auth_events async
  make FederationHandler._update_auth_events_and_context_for_auth async
  make FederationHandler.do_auth async
  make FederationHandler._prep_event async
  make FederationHandler._handle_new_event async
  ...

changelog.d/6837.misc

@@ -0,0 +1 @@
+Port much of `synapse.handlers.federation` to async/await.