reverse_proxy: Add local circuit breaker
reverse_proxy: Add local circuit breaker

This migrates a feature that was previously reserved for enterprise
users, according to https://github.com/caddyserver/caddy/issues/2786.

The local circuit breaker is a simple metrics counter that can cause
the reverse proxy to consider a backend unhealthy before it actually
goes offline, by measuring recent latencies over a sliding window.

Credit to Danny Navarro

l3dlp/caddy,abiosoft/caddy,mholt/caddy,l3dlp/caddy,mholt/caddy,abiosoft/caddy,l3dlp/caddy