Refactor session and connection handling in server. Fix QoS level 2 issues.

Source/MQTTnet/Server/Status/MqttSessionStatus.cs

@@ -0,0 +1,36 @@
+using System;
+using System.Threading.Tasks;
+
+namespace MQTTnet.Server.Status
+{
+    public class MqttSessionStatus : IMqttSessionStatus
+    {
+        private readonly MqttClientSession _session;
+        private readonly MqttClientSessionsManager _sessionsManager;
+
+        public MqttSessionStatus(MqttClientSession session, MqttClientSessionsManager sessionsManager)
+        {
+            _session = session ?? throw new ArgumentNullException(nameof(session));
+            _sessionsManager = sessionsManager ?? throw new ArgumentNullException(nameof(sessionsManager));
+        }
+
+        public string ClientId { get; set; }
+
+        public long PendingApplicationMessagesCount { get; set; }
+
+        public bool IsConnected { get; set; }
+    
+        public DateTime CreatedTimestamp { get; set; }
+
+        public Task DeleteAsync()
+        {
+            return _sessionsManager.DeleteSessionAsync(ClientId);
+        }
+        
+        public Task ClearPendingApplicationMessagesAsync()
+        {
+            _session.ApplicationMessagesQueue.Clear();
+            return Task.FromResult(0);
+        }
+    }
+}