Optimize device updates through a centralized update loop

[mwittig]

Implement a centralized update loop is desirable where each device can subscribe to the update topics in question (e.g. query the volume level) and the update handler can provide optimized queries which result in less callouts to the receiver.

[mwittig]

For the HTTP protctol handler I have already implemented an optimized update loop which works well. As newer receivers (since 2014, at least) provide the web interface I am not planning to implement an otimized update loop for TELNET. I'll add a note that the protocol should be switched HTTP if supported by the receiver.