Refactor ConnectedClient statistics API

Frameworks/MQTTnet.NetStandard/Server/IMqttClientSessionStatus.cs

@@ -0,0 +1,29 @@
+using System;
+using System.Threading.Tasks;
+using MQTTnet.Serializer;
+
+namespace MQTTnet.Server
+{
+    public interface IMqttClientSessionStatus
+    {
+        string ClientId { get; }
+
+        string Endpoint { get; }
+
+        bool IsConnected { get; }
+
+        MqttProtocolVersion? ProtocolVersion { get; }
+
+        TimeSpan LastPacketReceived { get; }
+
+        TimeSpan LastNonKeepAlivePacketReceived { get; }
+
+        int PendingApplicationMessagesCount { get; }
+
+        Task DisconnectAsync();
+
+        Task DeleteSessionAsync();
+
+        Task ClearPendingApplicationMessagesAsync();
+    }
+}