Keep a trailing AS set out of the origin and the peerings (closes #75)

A path that ends in an AS set of several members, such as
[64496, 64497, [64498, 64499]], now has no origin: its route is stored
with origin 0 (none), and no member of the set is in a peering. A set of
one member counts as that AS. An announcement with an empty path is
still not stored.

ASPath keeps such a set apart from the sequence and gives the origin.
The prefix and AS handlers take the origin from it; the peering handler
uses the sequence only. The stored AS path still lists the set's members
at the end.

The prefix and prefix length pages show origin 0 as no known origin, the
IP lookup asks no WHOIS lookup for it, and the README says what it means.

Model: opus-5-5

internal/database/database_internal_test.go

@@ -213,6 +213,33 @@ func TestGetIPInfoIgnoresShortRoutes(t *testing.T) {
 	lookup("2001:db8::1", "2001:db8::/32", 64502)
 }
 
+// TestGetIPInfoNeedsNoWHOISRefreshForOriginZero checks that the IP lookup asks
+// for no WHOIS lookup when the address's route has origin 0, no known origin,
+// and still asks for one when its origin AS has no WHOIS data.
+func TestGetIPInfoNeedsNoWHOISRefreshForOriginZero(t *testing.T) {
+	t.Parallel()
+
+	db, err := New(&config.Config{StateDir: t.TempDir()}, logger.New())
+	if err != nil {
+		t.Fatalf("failed to create database: %v", err)
+	}
+	defer func() { _ = db.Close() }()
+
+	writeOriginRoutes(t, db, map[string]int{"10.0.0.0/8": 0, "11.0.0.0/8": 64500})
+
+	for ip, want := range map[string]bool{"10.0.0.1": false, "11.0.0.1": true} {
+		info, err := db.GetIPInfo(ip)
+		if err != nil {
+			t.Fatalf("GetIPInfo(%s): %v", ip, err)
+		}
+
+		if info.NeedsWHOISRefresh != want {
+			t.Errorf("GetIPInfo(%s) asks for a WHOIS lookup of AS%d: %t, want %t",
+				ip, info.ASN, info.NeedsWHOISRefresh, want)
+		}
+	}
+}
+
 // TestLiveRouteTablesHoldOneRoutePerPrefixAndPeer checks that each live route
 // table refuses a second route from a peer for a prefix, even with another
 // origin AS. An upsert updates the peer's route before it would insert one, so