Store the routing table in the database (part of #122)

A new routes table holds one row per prefix and origin AS: the peer set
as the bitset's bytes, and the two kept routes, each with its peer's
number, AS path and time in seconds. A new route_peers table holds each
peer's number and address.

WriteRouteChanges writes one TakeChanges result in a transaction.
ReadRouteTable reads both tables into a new routing table, through the
new Table.Load, which records no change. It fails on a row that names a
peer number missing from route_peers. Nothing uses them yet.

Model: opus-5-5

internal/routetable/routetable_test.go

@@ -17,6 +17,7 @@ const (
 	peerA = "192.0.2.1"
 	peerB = "192.0.2.2"
 	peerC = "2001:db8::3"
+	peerD = "2001:db8::4"
 
 	asA = 64500
 	asB = 64501
@@ -475,6 +476,52 @@ func TestTakeChangesHoldsOnlyPairsThatChanged(t *testing.T) {
 	checkChanges(t, table, []change{}, map[string]uint{})
 }
 
+func TestLoad(t *testing.T) {
+	t.Parallel()
+
+	const otherV4 = "10.9.0.0/16"
+
+	v4 := netip.MustParsePrefix(prefixV4)
+	v6 := netip.MustParsePrefix(prefixV6)
+	other := netip.MustParsePrefix(otherV4)
+
+	// The routes of peerC and peerB to v4 are kept; peerA keeps only its bit.
+	// The two routes to v6 have the same time. The routes of other were
+	// withdrawn while peerA still carries it.
+	source := routetable.New(routetable.Params{})
+	announceAll(source, prefixV4, asA, 10, peerA, peerB, peerC)
+	announceAll(source, prefixV4, asB, 40, peerD)
+	source.Announce(v6, asA, routeOf(peerA, asA, 50))
+	source.Announce(v6, asA, routeOf(peerC, asA, 50))
+	announceAll(source, otherV4, asB, 10, peerA, peerB, peerC)
+	source.Withdraw(other, peerB)
+	source.Withdraw(other, peerC)
+
+	changes, peers := source.TakeChanges()
+	loaded := routetable.New(routetable.Params{PeerNumbers: peers})
+
+	// Loading a pair twice changes nothing.
+	for range 2 {
+		for _, change := range changes {
+			loaded.Load(change)
+		}
+	}
+
+	for _, prefix := range []netip.Prefix{v4, v6, other} {
+		got, want := loaded.Origins(prefix), source.Origins(prefix)
+		if !reflect.DeepEqual(got, want) {
+			t.Errorf("Origins(%s) = %+v after the load, want %+v", prefix, got, want)
+		}
+	}
+
+	if !reflect.DeepEqual(loaded.Counts(), source.Counts()) {
+		t.Errorf("Counts() = %+v after the load, want %+v",
+			loaded.Counts(), source.Counts())
+	}
+
+	checkChanges(t, loaded, []change{}, map[string]uint{})
+}
+
 // TestConcurrentUse has goroutines update one table while others read it. The
 // race detector, which make test runs, fails it when a method uses the table
 // without its lock.