Feature/rest calls

- Apologies for the large PR, fixing the tests took some refactoring
- To make the demux executors non-blocking, I added some become/unbecome logic 
- Moved the request work to the executors and they will only pass the request up to parent if they are idle. Not sure if this is ideal
- Return response codes from the api endpoints

I saw that you had created this message type in the protocol and I interpreted it as "forget that we have already demultiplexed this unit and requeue it". There is no implementation in master though and I wanted to check that with you.