#24 Multiple channels per client connection

[Hanno Braun]

The comm system should be able to provide multiple channels per client connection.
Per default, each client connection should only have one channel, called the main channel. Additional channels can be created by calling a method defined by the Client interface. For send operations, a channel can be given (but should default to main if not given).

Implementation:
To circumvent potential issues with firewalls, clients should be sent a message that causes them to connect to the server again. This behavior should be implemented in IoClient.
For the necessary communication, built-in message types should be used that have id values below zero.