new approach to handling Transport Masters when engine is restarted

Trust that ::reset() works for all transport masters, and call it when engine is stopped. This way
the transport masters are ready to be called again as soon as the engine restarts.

libs/ardour/engine_slave.cc

@@ -57,6 +57,12 @@ Engine_TransportMaster::check_backend()
 	}
 }
 
+void
+Engine_TransportMaster::reset (bool)
+{
+	_starting = false;
+}
+
 bool
 Engine_TransportMaster::locked() const
 {