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

Change the name of the variable from requestCode to request

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

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

- 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.

This reverts commit ac326babdf.

This reverts commit a38623cb5a.

- the Info level for publishing subscriptions is quite noisy
 - previously this wasn't a problem because subscription processing was buffered, now the message is logged for nearly every single un/subscription that is made
 - all other frequently recurring log events are also on level Verbose

This change kills several birds with one stone:
 - by performing the re-publishing of all subscriptions only at reconnect it fixes issue https://github.com/chkr1011/MQTTnet/issues/569 (retained messages are received again with every new subscription that is added)
 - not sending the subscriptions again with every (un)subscription is also a performance improvement if subscriptions are modified on a regular basis, because only the updated subscriptions are sent to the broker (previously, if you had 100 subscriptions and added a new one, 100 suscriptions were re-sent to the broker along with the new one, causing a significant delay)
 -until now subscriptions were sent to the broker only every ConnectionCheckInterval which caused unnecessary delays, and for request-response patterns could cause reponse messages to be missed due to the subscription delay. Now (un)subscriptions are published immediately

Subscriptions are now cleaned up at logout (after the connection stops being maintained). This is in line with the clearing of the _messageQueue in StopAsync

Explanatory note: the _subscriptionsQueuedSignal could ideally be a ManualResetEvent(Slim) but those do not offer an awaitable Wait method, so a SemaphoreSlim is used. This possibly causes a few empty loops in PublishSubscriptionsAsync due to the semaphore being incremented with every SubscribeAsync call but all subscriptions getting processed in one sweep, but those empty loops shouldn't be a problem