servewaf answers rproxy's question about each request and takes its report (closes #170)

`smallwebwaf servewaf` runs today's checks on the visitor's request each
question carries, answers rproxy, and passes nothing to an app. rproxy reports
every request servewaf answered. A report counts an allowed request's bytes and
writes servewaf's second log line. The table holds a copy of what a report
needs, never the request, so a report cannot reach a request being handled.

Judgement call: a question's body is read before any check; over 64 KiB gets 400.
Judgement call: a report's bytes and durations have ceilings, so they cannot overflow.
Judgement call: a `close` disposition for SWWAF_BAN_RESPONSE=close.

Model: opus-5-5

docs/metrics.md

@@ -79,3 +79,14 @@ By `file`:
   `smallwebwaf_alerts_failed_total`, `smallwebwaf_alerts_suppressed_total` and
   `smallwebwaf_alerts_dropped_total`;
 - Go's own `go_` metrics and the process's `process_` metrics.
+
+## `servewaf`
+
+`smallwebwaf servewaf` counts a denied request at its decision. It counts an
+allowed request when its report comes, with the status, bytes and times the
+report gives. These are not counted:
+
+- a report itself;
+- an allowed request whose report never comes;
+- a question refused with `400`;
+- a question whose body stopped arriving.