Report which public blocklists list an address in the IP lookup (closes #158)

A new package, internal/blocklist, downloads seven public lists on a
schedule: DShield, Feodo Tracker, FireHOL abusers and level 3, Spamhaus
DROP for IPv4 and IPv6, and the Tor exit list. It keeps the last good
copy of each in memory and on disk. A download sends back the
Last-Modified and ETag the publisher gave for the copy, also kept on
disk.

Every IP lookup result carries blocklists. An address with no live route
now gets a result too, without route fields, so the lookup no longer
answers 404. A successful answer carries Cache-Control, private for the
caller's own address. The address sets come from go4.org/netipx.

Model: opus-5-5

internal/database/models.go

@@ -92,12 +92,11 @@ type PrefixDistribution struct {
 //
 //nolint:tagliatelle // field names are the existing JSON API
 type IPInfo struct {
-	IP         string   `json:"ip"`
-	PTR        []string `json:"ptr,omitempty"`
-	Netblock   string   `json:"netblock"`
-	MaskLength int      `json:"mask_length"`
-	IPVersion  int      `json:"ip_version"`
-	NumPeers   int      `json:"num_peers"`
+	IP         string `json:"ip"`
+	Netblock   string `json:"netblock"`
+	MaskLength int    `json:"mask_length"`
+	IPVersion  int    `json:"ip_version"`
+	NumPeers   int    `json:"num_peers"`
 	// AS information
 	ASN         int    `json:"asn"`
 	ASName      string `json:"as_name,omitempty"`