Refactor MQTT handling

[gitseti]

Additional behavior changes:

• Removed the connect check, which returns a client directly when it’s already connected because it prevents users to do session takeovers in the shell. Instead they should use the switch command.
• When using multiple topics in the subscribe command, then only one subscribe is send instead of one subscribe for each topic.
• Remove shutdown hooks, so that client connections can be interrupted without the standard disconnect behavior -> On shell exit and subscribe interrupt the disconnect behavior stays the same
• Switch now can be used by only providing the client
• Improved publish command: multiple publishes by using multiple topics are now truly async instead of waiting for completion after each publish
• Failing and successful disconnects are now logged
• If the the will topic is missing connecting clients will now fail with an IllegalArgumentException instead of just logging a warning

TODO:

• Unit Tests
• Check all open TODO comments