Fix handling of PubRelMarker, missing inflightTimeouts.add()

PubRelMarkers should not be added to inflightWindow, and should not
cause the inflightSlots being decremented.
Also makes sure that non-pubRelMarker messages get added to the
inflightTimeouts.

broker/src/main/java/io/moquette/broker/Session.java

@@ -392,6 +392,7 @@ private void drainQueueToConnection() {
             }
             inflightSlots.decrementAndGet();
             int sendPacketId = mqttConnection.nextPacketId();
+            inflightTimeouts.add(new InFlightPacket(sendPacketId, FLIGHT_BEFORE_RESEND_MS));
 
             // Putting it in a map, but the retain is cancelled out by the below release.
             EnqueuedMessage old = inflightWindow.put(sendPacketId, msg);