wgengine,ipn/ipnlocal: remove Engine.PeerKeyForIP and the engine's peer route table

The engine kept its own longest-prefix-match table (peerByIPRoute),
rebuilt from the full peer list on every reconfig, to route outbound
packets and answer PeerKeyForIP. That's now the route manager's job:
LocalBackend already installs a PeerByIPPacketFunc backed by the
RouteManager's incrementally-maintained outbound table, so the
engine's copy was redundant state with redundant O(n peers) rebuild
work.

Delete the table, the PeerKeyForIP interface method, and the BART-only
default callback. LocalBackend's peerForIP now queries the
RouteManager's outbound table directly for the subnet-route and
exit-node fallback. Engines running without a LocalBackend (such as
wgengine/bench) must install their own outbound peer lookup, since the
device's standard AllowedIPs trie only covers peers that already
exist and can't lazily create them.

Updates #12542

Change-Id: I25100399e273ed6c2bb1f6136b7cd81bc83e7313
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/bench/wg.go

@@ -82,6 +82,17 @@ func setupWGTest(b *testing.B, logf logger.Logf, traf *TrafficGen, a1, a2 netip.
 	e1.SetFilter(filter.NewAllowAllForTest(l1))
 	e2.SetFilter(filter.NewAllowAllForTest(l2))
 
+	// There is no LocalBackend in this benchmark, so install trivial
+	// outbound peer lookups; without one, outbound packets can't
+	// lazily create their WireGuard peer.
+	k1pub, k2pub := k1.Public(), k2.Public()
+	e1.SetPeerByIPPacketFunc(func(dst netip.Addr) (_ key.NodePublic, ok bool) {
+		return k2pub, a2.Contains(dst)
+	})
+	e2.SetPeerByIPPacketFunc(func(dst netip.Addr) (_ key.NodePublic, ok bool) {
+		return k1pub, a1.Contains(dst)
+	})
+
 	var wait sync.WaitGroup
 	wait.Add(2)