Add asn= and prefix= tags from a routewatch instance (closes #204)

SWW_ROUTEWATCH_URL names the instance. The new heuristic routewatch_tags
asks it in the background about each client and tags the request. Answers
are kept in memory for an hour. A request without tags gives the reason in
routewatch_tags_missing, and it is never delayed or refused.

Model: opus-5-5

docs/request-log.md

@@ -171,6 +171,12 @@ Every one of these lines has:
     - `abuseipdb`.
 - `tags`: the request's tags, sorted, left out when it has none. Each tag and
   where it comes from is under "Request tags" in [`SPEC.md`](../SPEC.md).
+- `routewatch_tags_missing`: why the request got no tags from routewatch,
+  present only while `SWW_ROUTEWATCH_URL` is set:
+    - `no_answer_yet`: routewatch has not answered about this client yet;
+    - `request_failed`: a query to routewatch failed within the last minute, and
+      the process log says why;
+    - `no_route`: routewatch knows no live route for the address.
 - `offence`: `limit`, for a request that broke a limit.
 - `ban_expires`: when the client's ban ends, for a request that made one or was
   refused under one: a time, or `permanent`.

docs/settings.md

@@ -173,11 +173,19 @@ Every request gets tags from what `discriminator` knows of its client. See
   `exploitsignature`: a request gets that tag after the rate limits have counted
   it.
 
+- `SWW_ROUTEWATCH_URL` (default unset): the `http` or `https` URL of a
+  routewatch instance. `discriminator` asks it about each client, in the
+  background, and adds the tags `asn=<number>` and `prefix=<netblock>` from its
+  answer. While it is unset, nothing asks routewatch and no request gets a
+  `prefix=` tag.
+
 `SWW_TAG_DENY` and `SWW_TAG_LIMIT_PERCENT` accept tags in either case. A tag
 that no request can get stops the start. A request can get only these:
 
 - a name in `SWW_TAG_LISTS`;
 - an `asn=` or `country=` tag;
+- a `prefix=` tag with a netblock, while `SWW_ROUTEWATCH_URL` is set. It matches
+  only that exact announced netblock, such as `prefix=192.0.2.0/24`;
 - a `dnsbl=` tag of a zone in `SWW_DNSBL_ZONES`, in lower case, with a key
   masked as the `reputation` field shows it;
 - `abuseipdb`, while `SWW_ABUSEIPDB_KEY` is set;