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

It's a unit test to ensure the storage queue is drained.