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

By checking the Exception property it will consider the task exception as handled which means the finalizer won't throw an unhandled task exception.

Previously if there was an exception in the first task that is awaited the second task would not be awaited.

Now the task is always awaited and thus the exception in the task is handled.

Add a missing dispose in managed client.

This reverts commit 6d0e2c2022.

There isn't any code in MQTTnet that actually uses a cancellation token in WaitAsync, so this is more of a preventative thing than a bug fix.  The original code just checks if the task was completed, not whether it was cancelled.  If the process is cancelled immediately before the call to WaitAsync, it'll return as normal (https://referencesource.microsoft.com/#mscorlib/system/threading/SemaphoreSlim.cs,612) rather than throw a cancellation exception.  This change will ensure we only return the releaser if the wait actually ran to completion rather than exited early due to cancellation.  I've tested this, and it properly throws a cancellation exception later on.

Sometimes, TryPublishQueuedMessageAsync would try to remove a message from the storage queue before PublishAsync added it to the storage queue, resulting in a message being stuck in the storage queue forever.  Switched the message queue lock to an async lock and synchronized the storage queue updates with the message queue updates.