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

Five failed asks to waf within five seconds open the breaker. An ask
fails when it gets no answer in time, slow answers included, or an
answer rproxy cannot pass on. Every usable answer clears the failures
and closes the breaker. While open, a request takes
SWW_RPROXY_WAF_BACKEND_NO_ANSWER_DISPOSITION without asking waf, except
one ask a second. 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/request-log.md

@@ -238,7 +238,8 @@ meanings where they differ:
 - `disposition`: what was done with the request: `allow`, `deny` or `close`. It
   is `waf`'s answer, or the value of
   `SWW_RPROXY_WAF_BACKEND_NO_ANSWER_DISPOSITION`. It is left out when:
-    - `waf` was not asked, as for the health check;
+    - `waf` was not asked and `SWW_RPROXY_WAF_BACKEND_NO_ANSWER_DISPOSITION` did
+      not apply, as for the health check;
     - the visitor went away while `waf` was being asked;
     - `waf`'s answer had no valid `X-Sww-Disposition`;
     - `rproxy` could not pass on `waf`'s answer, and answered `502`.

docs/settings.md

@@ -453,10 +453,9 @@ start, and an invalid value stops the start. Any can be given as a file, as a
   for `waf`'s answer, to a question and to a report. A time in milliseconds;
   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.
+  with a request when `waf` gives no answer in time, 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