Simplistic registry to aid in endpoint negotiation

updated serial to take advantage, (minimally) refactored rf12demodecode to take advantage of "instancing" of tags.
(housemon/rf12demodecode PULL to come behind this.)

Please try it out, I use (a version) of this successfully, that adds features such as TTL reaping of endpoints etc, but kept that back for now as dont want to be accused of feature creep 

Key features of this are:-
Simplistic client registration (almost all hidden). Pretty generic, can be used by other input systems. Can easily expand to handle non-line orientated devices via better endpoint control. One you have registered, it sits out the way (this version does not watch anything).

In a nutshell, send mqtt message to register, await a response, use endpoint+timestamp to write, listen on endpoint for read (from standpoint of device).
Guaranteed to get a structured endpoint of format <root>/<tag>/<taginst>