rproxy: circuit breaker for asks to waf (closes #392)

Five asks that get no answer in time within five seconds open the
breaker. While open, a request takes
SWW_RPROXY_WAF_BACKEND_NO_ANSWER_DISPOSITION without asking waf, except
one ask a second; the first answer closes it. Slow answers count as
failures. SPEC.md describes it.

The sww test that stops and restarts waf now waits for the breaker to
close before it expects the ban again.

Model: opus-5-5

README.md

@@ -26,7 +26,7 @@ shipped in one image, with two commands that do this together:
 - `sww rproxy` is a small reverse proxy in front of one app. It sits behind a
   TLS-terminating proxy, such as traefik, nginx or Caddy. It asks `waf` about
   each request, passes the allowed ones to the app, and passes the app's answers
-  back. It keeps nothing between requests and writes nothing to disk.
+  back. It writes nothing to disk.
 - `sww waf` decides. It runs the checks and keeps the bans, the counters and
   each client's history. One `waf` serves many sites: a client banned through
   one site is refused on all of them, and its requests on all of them count