Fix ManagedMqttClient missing call to unsubscribe topics

Fix infinite loop when the client closes the connection

Fix the algorithm for decoding the body length

See: https://docs.oasis-open.org/mqtt/mqtt/v3.1.1/errata01/os/mqtt-v3.1.1-errata01-os.html#_Toc442180732

Use asynchronous methods for blocking operations

Otherwise, Worker Threads from the .NET Threadpool are blocked while waiting for the next client message, and while waiting for the next message to send.