Reorder client registration

[michaelkaye]

There's a possible issue atm where clients register but stop polling for updates by the time they're allocated to a job.

It would be better if clients sat in a "register" loop until they were needed, and only then moved onto setup and then start polling for requests.

This reduces things like lengths of videos stored and gets better timings on how long tests run, and also gives us two piles of clients - one pile waiting for work and the other doing the work.

TBD: how the API looks; how things move between states.