Fix for #762 - thread pool leak

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.

Better handling of AssignedClientIdentifier - chkr1011/MQTTnet#745

fixed #743

Fix Client SslStream CRL verification

Add a missing dispose in managed client.

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This reverts commit 6d0e2c2022.

This reverts commit 6c53677e7d.

Add Server certificate password and client certificate builders