statsd-proy: reset/delete state of metrics on statsd instances when ring members change [WIP]

Sending this PR over on req from @draco2003. This code does NOT seem to work as intended. I'm not sure exactly why, I suspect perhaps there's a race condition due to metrics coming in at such a high rate, but it still seems like this approach should work.

I didn't spend too much time on it, however, as simply setting `deleteIdleStats: true` on the statsd side achieved roughly the same goal. Still, I think it would be desirable to have a working reset-on-ring-change function.

I could help test if someone has a better idea on how to approach this.