Jobs can be processed and left in the `wait` state

Created by: bradvogel

@manast Following the discussion in https://github.com/OptimalBits/bull/pull/359, I found a hazard that I think we should address before releasing a new version: a job can be processed an still left in wait. Here's how:

Time	Process A	Process B
1	In the regular Bull run loop of Process A, getNextJob moves a job from wait to active
2		In Process B, moveUnlockedJobsToWait happens to run and pick up the job in step 1, and moves it back to wait
3	Process A gets lock on the job (that is now in 'wait') and begins to process it

If there is no other process around to move the job back to active then Process A will complete the job and leave it in wait. This can lead to a data inconsistency.