Use latest language features (#2176)

* Make use of language features line AsyncDispose, init etc.

* Fix build errors

* Improve Unit Tests

* Improve Unit Tests

* Improve Unit Tests

* Update samples

* Remove obsolete benchmarks

* Code cleanup

* Improve unit tests

* Improve Unit Tests

* Fix Unit Tests

* Improve Unit Tests

* Improve Unit Tests

* Restore unit tests

* Exclude load tests from unit tests

Source/MQTTnet.Benchmarks/MqttTcpChannelBenchmark.cs

@@ -2,7 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-using System;
 using System.Buffers;
 using System.Reflection;
 using System.Threading;