MQTT: reduce initializing->ready fallback timer timeout to 4s

Wait no longer than 4s to receive all "initial" messages from the
broker, before transitioning to Ready state.

src/qt/ve_qitems_mqtt.cpp

@@ -134,7 +134,7 @@ VeQItemMqttProducer::VeQItemMqttProducer(
 		});
 
 	mReadyStateFallbackTimer->setSingleShot(true);
-	mReadyStateFallbackTimer->setInterval(10000);
+	mReadyStateFallbackTimer->setInterval(4000);
 	connect(mReadyStateFallbackTimer, &QTimer::timeout,
 		this, [this] {
 			mReadyStateTimer->stop();