Reduce the performance cost of Memory<T>.getSpan()

Aspnetcore3bugfixes

DIspose semaphore in MqttChannelAdapter

Added MqttClientUnsubscribeOptionsBuilder

Added receiveTracker to detect whether messages are received from broker.

Add .NET 3.1 target

Add .NET Core 3.1 target that uses ASP.NET Core framework reference instead of NuGet package references

Allow access to full packet on failed connect

Change the name of the variable from requestCode to request

Lost a change while moving from my developement branch to here.

bugfixes

There are some systems that return information on a failed exception and thus the client wants access to the full set of ACK information.  Specifically the user properties.

Return the response not the request set of user properties

Bugfixes

Support existing session with ManagedMqttClient.

Added IMqttRetainedMessagesManager

- adding client and server wrappers to verify the clientid starts with the testname
- adding disposable baseclass
- clear disconnect handler from client so it does not try to reconnect after it is disposed
- moved code from dispose to cleanup in case it is called in a stop method as dispose may only be called once while start / stop may be called multiple times

* Added interface IMqttRetainedMessagesManager to allow for different
retained messages manager implementations. The interface copies almost
exactly the current MqttRetainedMessagesManager implementation.

* Added IMqttRetainedMessagesManager.Start() to configure the provided
IMqttRetainedMessagesManager outside the constructor. This method returns
Task because some implementations that use external storage may be slow on
initialization (e.g., connect to a database).

* Modified MqttRetainedMessagesManager to implement new interface.