TrialViewSet.dequeue() has a race condition

[carmichaelong]

Multiple backend workers can pull the same trial since dequeue() is non-blocking, and there are two API tasks running Django for load balancing. This may be causing an intermittent issue (now rarer with less DB pressure) where a trial will appear as an error to a user initially, as one backend worker will fail to upload results (its URL is likely changed with the second worker) but the second one will succeed. This could cause a user to believe their calibration or neutral trial failed due to a setup issue, since the first worker will return an error.