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

README.md

@@ -261,6 +261,7 @@ Every setting the code reads, with its default:
 | `SWW_TAG_LISTS`                          | empty                                                                          | Address lists that each add a tag to their clients' requests, as `name=URL` pairs, such as `torexit=https://example.com/tor.txt`.                                 |
 | `SWW_TAG_DENY`                           | empty                                                                          | Tags whose requests are refused, such as `torexit,country=CN`.                                                                                                    |
 | `SWW_TAG_LIMIT_PERCENT`                  | empty                                                                          | Tags whose clients get a percentage, from 1 to 99, of every request and byte limit, such as `vpn:50,asn=14061:25`; the lowest applies.                            |
+| `SWW_ROUTEWATCH_URL`                     | unset                                                                          | The URL of a routewatch instance, which gives each client's `asn=` and `prefix=` tags. Unset, nothing asks routewatch.                                            |
 | `SWW_ASN_BYTES_PERCENT`                  | empty                                                                          | A percentage of the byte limits alone for the AS numbers listed, in place of their `asn=` tag's percentage.                                                       |
 | `SWW_COUNTRY_BYTES_PERCENT`              | empty                                                                          | A percentage of the byte limits alone for the countries listed, in place of their `country=` tag's percentage.                                                    |
 | `SWW_UNKNOWN_LIMIT_PERCENT`              | `100`                                                                          | The percentage of every limit a client without a country gets.                                                                                                    |
@@ -562,7 +563,9 @@ the `tags` field of its log line:
   or `vpn=mullvad`;
 - `crowdsec`, `dnsbl=<zone>` and `abuseipdb`, when that reputation source lists
   the client;
-- `exploitsignature`, when a rule file or the Core Rule Set matched the request.
+- `exploitsignature`, when a rule file or the Core Rule Set matched the request;
+- `asn=<number>` and `prefix=<netblock>`, from a routewatch instance, while
+  `SWW_ROUTEWATCH_URL` is set.
 
 `SWW_TAG_DENY` refuses requests by tag, and `SWW_TAG_LIMIT_PERCENT` lowers a
 client's limits by tag. See "Request tags" in [`SPEC.md`](SPEC.md).