Add support for multi-priority workqueues

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.

changes/multi-priority

@@ -0,0 +1,5 @@
+  o Minor features (relay, thread pool):
+    - Allow background work to be queued with different priorities, so
+      that a big pile of slow low-priority jobs will not starve out
+      higher priority jobs. This lays the groundwork for a fix for bug
+      22883.