Per-client request rate limits over a minute, an hour and a day (closes #43)

Each client, one IPv4 address or one IPv6 /64, is counted in two buckets
per window, the earlier weighted by how much of it the window covers; at
most 20,000 clients are kept, least recently seen dropped first. A
request over SWWAF_RATE_LIMIT_PER_MINUTE, _HOUR or _DAY (1000, 10000,
50000, or off) gets 429 before reaching the app. Refused requests count,
413s included. A clock set back over a second behind a bucket's start
restarts that window. The log line gains limit_hit and the action
rate_limited.

Deviation from SPEC.md, per the issue: the 20,000 bound and /64 are fixed.
Judgement call: golang-lru/v2 holds the table; httprate does not count refused requests.
Deviation: go.mod and go.sum hand-written; no make target tidies them.

Model: opus-5-5

internal/requestlog/requestlog.go

@@ -23,6 +23,9 @@ const (
 	// ActionUpstreamError is a request the app could not be reached
 	// for, or whose answer could not be passed on.
 	ActionUpstreamError = "upstream_error"
+	// ActionRateLimited is a request refused because it took its client
+	// over a rate limit, or came while the client was over one.
+	ActionRateLimited = "rate_limited"
 )
 
 // timeLayout is RFC 3339 with milliseconds.
@@ -49,6 +52,9 @@ type Line struct {
 	Referer        string `json:"referer"`
 	UserAgent      string `json:"user_agent"`
 	Action         string `json:"action"`
+	// LimitHit is the window whose rate limit the request went over:
+	// minute, hour or day.
+	LimitHit string `json:"limit_hit,omitempty"`
 	// Aborted is true when the client went away early.
 	Aborted bool `json:"aborted,omitempty"`
 	// DurationTotal and DurationUpstreamTotal are in milliseconds.