Improve the submitter code

[tomverran]

The submitter uses a Fiber to run a background task which means we have to use a Semaphore to ensure it isn't stopped between removing data from the queue & submitting it when the app shuts down.

My theory is we could operate at a slightly higher level and use FS2 for this but it needs some careful consideration to ensure we don't introduce any new race conditions. I have a branch I can produce upon request.