Support existing session with ManagedMqttClient.

Added IMqttRetainedMessagesManager

* 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.

Replace raw bytes with x509 certificate to allow specify passwords and flags #2

Revert "Replace raw bytes with x509 certificate to allow specify passwords and flags"

This reverts commit ac326babdf.

This reverts commit a38623cb5a.

Replace raw bytes with x509 certificate to allow specify passwords and flags

Fix for #785 - Skip client disconnection process if unauthorized

Reduce subscription publishing loglevel in ManagedMqttClient

- 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

Separate current and reconnect subscriptions in managed client

Adapt test assembly paths in buildscript

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

Remove if clause that stopped handling of tasks with exceptions

Fix for #762 - thread pool leak