Added option for how many subscriptions to send in a single call to t… (#1209)

* Added option for how many subscriptions to send in a single call to the server.  aws iotcore limits this to 8

* Split subscription messages up to 8 at a time when reconnecting

Source/MQTTnet.Extensions.ManagedClient/ManagedMqttClientOptions.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using MQTTnet.Client.Options;
 using MQTTnet.Server;
 
@@ -17,5 +17,7 @@ namespace MQTTnet.Extensions.ManagedClient
         public int MaxPendingMessages { get; set; } = int.MaxValue;
 
         public MqttPendingMessagesOverflowStrategy PendingMessagesOverflowStrategy { get; set; } = MqttPendingMessagesOverflowStrategy.DropNewMessage;
+
+        public int? MaxSubcribeUnsubscribeMessagesAtOnce { get; set; } = null;
     }
 }