This commit responds to the comments on PR #85192 that were not yet
addressed at the time it merged, apart from the one fixed in PR
Generalized fairqueuing to allow for zero queues, to support a
priority level that limits concurrency but does no queuing.
Kubernetes-commit: b123a43e7117e977606bacd31d77f4a30d2ed212
(1) Replaced random-looking assortment of counter increments and
decrements with something hopefully more principalled-looking. Most
importantly, introduced the MutablePromise abstraction to neatly wrap
up the complicated business of unioning multiple sources of
unblocking.
(2) Improved debug logging.
(3) Somewhat more interesting test cases, and a bug fix wrt round
robin index.
Kubernetes-commit: 1c31b2bdc65377f502c2306dbdf32a802eb1afb7
Mike Spreitzer