Add first classes for persisted session support.

Source/MQTTnet/Server/IMqttServerOptions.cs

@@ -21,6 +21,8 @@ namespace MQTTnet.Server
         MqttServerTcpEndpointOptions DefaultEndpointOptions { get; }
         MqttServerTlsTcpEndpointOptions TlsEndpointOptions { get; }
 
-        IMqttServerStorage Storage { get; }        
+        IMqttServerStorage Storage { get; }       
+        
+
     }
 }

Source/MQTTnet/Server/IMqttServerPersistedSession.cs

@@ -0,0 +1,22 @@
+using System;
+using System.Collections.Generic;
+
+namespace MQTTnet.Server
+{
+    public interface IMqttServerPersistedSession
+    {
+        string ClientId { get; }
+
+        IDictionary<object, object> Items { get; }
+
+        IList<TopicFilter> Subscriptions { get; }
+
+        MqttApplicationMessage WillMessage { get; }
+
+        uint? WillDelayInterval { get; }
+
+        DateTime? SessionExpiryTimestamp { get; }
+
+        IList<MqttQueuedApplicationMessage> PendingApplicationMessages { get; }
+    }
+}

Source/MQTTnet/Server/IMqttServerPersistedSessionsStorage.cs

@@ -0,0 +1,11 @@
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+
+namespace MQTTnet.Server
+{
+    public interface IMqttServerPersistedSessionsStorage
+    {
+        Task<IList<IMqttServerPersistedSession>> LoadPersistedSessionsAsync();
+    }
+}

Source/MQTTnet/Server/MqttClientConnection.cs

@@ -229,7 +229,7 @@ namespace MQTTnet.Server
                 }
                 else
                 {
-                    _logger.Error(exception, "Client '{0}': Unhandled exception while receiving client packets.", ClientId);
+                    _logger.Error(exception, "Client '{0}': Error while receiving client packets.", ClientId);
                 }
 
                 StopInternal();

Source/MQTTnet/Server/MqttClientSubscriptionsManager.cs

@@ -9,7 +9,7 @@ namespace MQTTnet.Server
 {
     public class MqttClientSubscriptionsManager
     {
-        private readonly Dictionary<string, MqttQualityOfServiceLevel> _subscriptions = new Dictionary<string, MqttQualityOfServiceLevel>();
+        private readonly Dictionary<string, TopicFilter> _subscriptions = new Dictionary<string, TopicFilter>();
         private readonly MqttClientSession _clientSession;
         private readonly IMqttServerOptions _serverOptions;
         private readonly MqttServerEventDispatcher _eventDispatcher;
@@ -64,7 +64,7 @@ namespace MQTTnet.Server
                 {
                     lock (_subscriptions)
                     {
-                        _subscriptions[finalTopicFilter.Topic] = finalTopicFilter.QualityOfServiceLevel;
+                        _subscriptions[finalTopicFilter.Topic] = finalTopicFilter;
                     }
 
                     await _eventDispatcher.HandleClientSubscribedTopicAsync(_clientSession.ClientId, finalTopicFilter).ConfigureAwait(false);
@@ -90,7 +90,7 @@ namespace MQTTnet.Server
                 {
                     lock (_subscriptions)
                     {
-                        _subscriptions[topicFilter.Topic] = topicFilter.QualityOfServiceLevel;
+                        _subscriptions[topicFilter.Topic] = topicFilter;
                     }
 
                     await _eventDispatcher.HandleClientSubscribedTopicAsync(_clientSession.ClientId, topicFilter).ConfigureAwait(false);
@@ -158,7 +158,7 @@ namespace MQTTnet.Server
                         continue;
                     }
 
-                    qosLevels.Add(subscription.Value);
+                    qosLevels.Add(subscription.Value.QualityOfServiceLevel);
                 }
             }