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

docs/settings.md

@@ -454,9 +454,10 @@ start, and an invalid value stops the start. Any can be given as a file, as a
   never `off`.
 - `SWW_RPROXY_WAF_BACKEND_NO_ANSWER_DISPOSITION` (default `allow`): what is done
   with a request when `waf` cannot be connected to, does not answer at all, or
-  does not answer within `SWW_RPROXY_WAF_BACKEND_TIMEOUT_MS`. `allow` passes it
-  to the app, and `deny` answers `503`. It does not apply to an answer `waf`
-  gives, such as an error.
+  does not answer within `SWW_RPROXY_WAF_BACKEND_TIMEOUT_MS`, and while the
+  circuit breaker for asks to `waf` is open. `allow` passes it to the app, and
+  `deny` answers `503`. It does not apply to an answer `waf` gives, such as an
+  error.
 - `SWW_RPROXY_MAX_REQUEST_SIZE` (default `100M`): the largest request body. A
   size or `off`.
 - `SWW_RPROXY_MAX_RESPONSE_SIZE` (default `1G`): the largest response body. A