We were decrementing activeJobCount after removing the job from the
queue, which could cause other threads in the pool to preempt us before
the decrement, causing them to spin forever trying to get a non-existant
job, until the decrement actually happened.
Now we always decrement first and fix-up the count if we couldn't get
a job from the queues. The race is inverted, and doesn't cause threads
to spin a long time.
fixes b/201100123
19b0ad2605