MINOR: proxy: maintain per-state counters of listeners

MINOR: proxy: maintain per-state counters of listeners

The proxy state tries to be synthetic but that doesn't work well with
many listeners, especially for transition phases or after a failed
pause/resume.

In order to address this, we'll instead rely on counters of listeners in
a given state for the 3 major states (ready, paused, listen) and a total
counter. We'll now be able to determine a proxy's state by comparing these
counters only.