Heartbeat Protocol

This is a first draft for the heartbeat protocol. Some open questions or discussion points:

• As per #12 we should always heartbeat, independent of satellite importance or state
• Right now the protocol contains the actual state codes - which is not so nice. Just as CSCP does not contain the commands, I think CHP should not define the states. What do you think?
• Right now the protocol only defines lives counter and state cache for each subscription but no action to be taken. So the actual "when heartbeat goes missing we go SAFE is not in - just like the above point I think this is the way.
• We should then have a separate document (part of API docs?) that define all the states and this logic in technical terms to serve as reference for the implementation?
• Should we really "hard-code" Timeouts and lives values in the protocol?

This closes #10 (closed)