Refactor front end code so message decoding is externally callable

By making the message decoding function externally callable we gain the following benefits:

* The code becomes unit testable.
* The code can be fed messages from the raw_messages table for reprocessing those messages.
* In the case of MQTT messages the code can be dynamically loaded and called based upon topic name patterns so we only need one MQTT receiving process.

The decoding function should take 1 message 'as received' and return an array of decoded messages in IoTa's internal format.

The decoding function will be responsible for determining the physical device for each returned message because this is source/provider specific.