This change makes cpuworker and test_workqueue no longer need to
include event2/event.h. Now workqueue.c needs to include it, but
that is at least somewhat logical here.
Each piece of queued work now has an associated priority value; each
priority goes on a separate queue.
With probability (N-1)/N, the workers will take work from the highest
priority nonempty queue. Otherwise, they'll look for work in a
queue of lower priority. This behavior is meant to prevent
starvation for lower-priority tasks.
I grepped and hand-inspected the "it's" instances, to see if any
were supposed to be possessive. While doing that, I found a
"the the", so I grepped to see if there were any more.
Previously I used one queue per worker; now I use one queue for
everyone. The "broadcast" code is gone, replaced with an idempotent
'update' operation.
