So that errors can be detected before resolving concurrency shares
into concurrency counts.
Kubernetes-commit: 1e170637c3ce6c4ccd378275d9e52192f4be12b7
(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
Aaron Prindle