oauth: Track total call count during a client flow

oauth: Track total call count during a client flow

Tracking down the bugs that led to the addition of comb_multiplexer()
and drain_timer_events() was difficult, because an inefficient flow is
not visibly different from one that is working properly. To help
maintainers notice when something has gone wrong, track the number of
calls into the flow as part of debug mode, and print the total when the
flow finishes.

A new test makes sure the total count is less than 100. (We expect
something on the order of 10.) This isn't foolproof, but it is able to
catch several regressions in the logic of the prior two commits, and
future work to add TLS support to the oauth_validator test server should
strengthen it as well.

Backpatch-through: 18
Discussion: https://postgr.es/m/CAOYmi+nDZxJHaWj9_jRSyf8uMToCADAmOfJEggsKW-kY7aUwHA@mail.gmail.com