Don't give out job ids to workers, but `tokens`, which serve as an
indirection. This way, restarting composer won't confuse it when a stray
worker returns a result for a job that was still running. Also,
artifacts are only moved to the final location once a job finishes.
This change breaks backwards compatibility, but we're not yet promising
a stable worker API to anyone.
This drops the transition tests in server_test.go. These don't make much
sense anymore, because there's only one allowed transition, from running
to finished. They heavily relied on job slot ids, which are not easily
accessible with the `TestRoute` API. Overall, adjusting this seemed like
too much work for their benefit.
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