Fix #621 fix job re-queueing on hard shutdown (e.g. SIGTERM).

Jobs in process during a hard shutdown were not properly sent back to redis resulting in the jobs being permanently lost.

Steps to reproduce:
1) enqueue an infinitely running job, see "busy" workers go to 1
2) `killall -TERM -m sidekiq`
3) expect "enqueued" to be 1, but it is actually 0

This is particularly noticeable when scaling down workers on Heroku.