Look clients up in the IPinfo Lite file with SWWAF_LOOKUP_SOURCE=file (closes #22)

SWWAF_LOOKUP_SOURCE=file looks every client up in the file
SWWAF_LOOKUP_DB_PATH names, without GeoJS. file without the path, the
path with another source, or a file that cannot be read stops the start.
The file is read whole into memory, so overwriting it in place cannot
disturb a lookup, and read again 2 seconds after its last change; a
replacement that cannot be read is logged, counted and sent as a
file_error alert, and the old one stays in use. Metrics give when it
was read and the failed reads. Tests write their databases through
internal/lookup/lookuptest.

Deviation: go.mod and go.sum written by hand; go runs only through make.
Judgement call: the 2-second wait, as the rule files have.

Model: opus-5-5

internal/ratelimit/ratelimit.go

@@ -68,8 +68,8 @@ type History struct {
 	LastSeen  time.Time `json:"last_seen"`
 	// ASN, ASName and Country are the client's AS number, AS name and
 	// country as last looked up, each empty when the lookup could not
-	// find it, and LookedUp is when GeoJS gave that answer; all are empty
-	// while the client never was looked up.
+	// find it, and LookedUp is when the lookup gave that answer; all are
+	// empty while the client never was looked up.
 	ASN      string    `json:"asn,omitempty"`
 	ASName   string    `json:"as_name,omitempty"`
 	Country  string    `json:"country,omitempty"`
@@ -230,7 +230,8 @@ func (l *Limiter) AddToHistory(client netip.Prefix, now time.Time, r Request) {
 }
 
 // AddLookup gives client's history its AS number, AS name and country, as
-// GeoJS gave them at lookedUp, if the table of clients holds the client.
+// the lookup gave them at lookedUp, if the table of clients holds the
+// client.
 // It does not make the client the most recently seen.
 func (l *Limiter) AddLookup(
 	client netip.Prefix, lookedUp time.Time, asn, asName, country string,