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

README.md

@@ -339,6 +339,9 @@ pinned.
   [`docs/settings.md`](docs/settings.md)).
 - The state files live in `/var/lib/smallwebwaf`. A volume mounted there keeps
   them across a deploy.
+- `smallwebwaf servewaf` runs the same checks for the sites that ask it about
+  each request, and passes nothing to an app (see "`servewaf`" in
+  [`SPEC.md`](SPEC.md)).
 - `docker stop` stops both processes at once. `smallwebwaf` gives the requests
   in progress five seconds to finish, so the app must keep answering for five
   seconds after `SIGTERM`.
@@ -351,6 +354,8 @@ what each does.
 ## TODO
 
 - Planned, as "Planned, not built" in [`SPEC.md`](SPEC.md) describes:
+    - `smallwebwaf rproxy`, the reverse proxy in front of each app that asks
+      `smallwebwaf servewaf` about each request;
     - sharing bans between the instances of several apps;
     - sending the log over RELP.