close sockets when stopping federation (#546)

when we stop a federation server, we need to close the active sockets too,
otherwise they hang around like zombies.

mmm, brainz

tests/50federation/35room-invite.pl

@@ -302,6 +302,13 @@ sub do_v2_invite_request
                # error on make_leave
                $federation_server->close();
 
+               # close any connected sockets too, otherwise synapse will
+               # just reuse the connection.
+               foreach my $child ( $federation_server->children() ) {
+                  log_if_fail "closing", $child;
+                  $child->close();
+               }
+
                return matrix_leave_room( $user, $room_id );
             }
             else {