Dequeue atomic

[AlbertoCasasOrtiz]

Fixes #267.

Most changes are due to indentation. Real changes are the addition of atomic to make everything a single transaction:

with transaction.atomic():

Select for update, which locks the trial so other workers cannot modify or lock them. If already locked by another worker, it skips it.

.select_for_update(skip_locked=True)

And a status check that, if for any reason a worker gets a trial already locked and in a non-available status, will skip it:

if trial.status not in ["stopped", "reprocess"]:
    raise Http404

[AlbertoCasasOrtiz]

@carmichaelong I just fixed conflicts on this branch