Remove wrong setter.

Build/MQTTnet.nuspec

@@ -11,6 +11,7 @@
     <requireLicenseAcceptance>false</requireLicenseAcceptance>
     <description>MQTTnet is a high performance .NET library for MQTT based communication. It provides a MQTT client and a MQTT server (broker).</description>
     <releaseNotes>* [Server] Added new method which exposes all retained messages.
+* [Server] Removed (wrong) setter from the server options interface.
     </releaseNotes>
     <copyright>Copyright Christian Kratky 2016-2018</copyright>
     <tags>MQTT Message Queue Telemetry Transport MQTTClient MQTTServer Server MQTTBroker Broker NETStandard IoT InternetOfThings Messaging Hardware Arduino Sensor Actuator M2M ESP Smart Home Cities Automation Xamarin</tags> 

Source/MQTTnet/Server/IMqttServerOptions.cs

@@ -14,7 +14,7 @@ namespace MQTTnet.Server
         Action<MqttConnectionValidatorContext> ConnectionValidator { get; }
         Action<MqttSubscriptionInterceptorContext> SubscriptionInterceptor { get; }
         Action<MqttApplicationMessageInterceptorContext> ApplicationMessageInterceptor { get; }
-        Action<MqttClientMessageQueueInterceptorContext> ClientMessageQueueInterceptor { get; set; }
+        Action<MqttClientMessageQueueInterceptorContext> ClientMessageQueueInterceptor { get; }
 
         MqttServerTcpEndpointOptions DefaultEndpointOptions { get; }
         MqttServerTlsTcpEndpointOptions TlsEndpointOptions { get; }