This teaches glance-api how to do async threading things when it is
running in pure-WSGI mode. In order to do that, a refactoring of things
that currently depend on eventlet is required.
It adds a [wsgi]/task_pool_threads configuration knob, which is used
in the case of pure-WSGI and native threads to constrain the number
of threads in that pool (and thus the task parallelism). This will
allow tuning by the operator, but also lets us default that to just
a single thread in the backport of these fixes so that we can avoid
introducing a new larger footprint in the backport unexpectedly.