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 guess that you could make sure that the `FakeDemultiplexer` blocks for some time, so that the future does not return immediately and then you could test the behavior while that locks?