Returns True if work piece is unclaimed.

def is_unclaimed(work):
  """Returns True if work piece is unclaimed."""
  if work['is_completed']:
    return False
  cutoff_time = time.time() - MAX_PROCESSING_TIME
  if (work['claimed_worker_id'] and
      work['claimed_worker_start_time'] is not None
      and work['claimed_worker_start_time'] >= cutoff_time):
    return False
  return True