Bad get() performance with many jobs

[alecgibson]

I know this library is no longer maintained. We've moved onto our own fork, but just documenting this here because we spent a day hunting down this performance issue.

This library has bad get() performance when there are lots of queued jobs.

This is because:

• get() queries on deleted and visible, which has an index
• but it sorts on _id, which can't make use of the above index

The trivial fix is to change it to sort by visible, which is in the index and should give similar (if not better?) results, presenting the job that was made visible first (as opposed to the job that was created first).