Feature/mqttv5 subscriptions server (#1246)

* Add support for RetainHandling.

* Add support for No Local.

* Add support for _SubscriptionIdentifier_.

* Add more Unit Tests.

* Update docs.

Source/MQTTnet/PacketDispatcher/IMqttPacketAwaitable.cs

@@ -0,0 +1,16 @@
+using MQTTnet.Packets;
+using System;
+
+namespace MQTTnet.PacketDispatcher
+{
+    public interface IMqttPacketAwaitable : IDisposable
+    {
+        MqttPacketAwaitableFilter Filter { get; }
+        
+        void Complete(MqttBasePacket packet);
+
+        void Fail(Exception exception);
+
+        void Cancel();
+    }
+}