wgengine/netlog: stop using netmap.NetworkMap type, use LocalBackend

The Logger previously took a *netmap.NetworkMap at Startup and on every
ReconfigNetworkMap call, denormalizing it into per-IP and self lookup
maps. That denormalization is O(n) over all peers and ran on every
netmap update, contributing to the broader quadratic behavior we want
to eliminate when a single peer is added or removed.

Instead, this makes netlog ask LocalBackend (well, nodeBackend) for
the info it needs, letting us remove the netmap.NetworkMap type
entirely from the netlog package.

This is a dependency to removing the netmap.NetworkMap type from
upstream callers, like wgengine.Engine in general.

Updates #12542

Change-Id: Ib5f2de96e788a667332c0a6f7ac833b3d0053b5c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/wgengine.go

@@ -16,6 +16,7 @@ import (
 	"tailscale.com/types/key"
 	"tailscale.com/types/netmap"
 	"tailscale.com/wgengine/filter"
+	"tailscale.com/wgengine/netlog"
 	"tailscale.com/wgengine/router"
 	"tailscale.com/wgengine/wgcfg"
 	"tailscale.com/wgengine/wgint"
@@ -164,6 +165,13 @@ type Engine interface {
 	// look up which peer should handle an outbound packet by destination IP.
 	SetPeerByIPPacketFunc(func(netip.Addr) (_ key.NodePublic, ok bool))
 
+	// SetNetLogNodeSource installs the [netlog.NodeSource] used by the engine's
+	// network logger to look up Tailscale node info on demand.
+	//
+	// It is expected to be called once during LocalBackend construction,
+	// before any [Engine.Reconfig] call that starts up the network logger.
+	SetNetLogNodeSource(netlog.NodeSource)
+
 	// SetPeerSessionStateFunc installs a callback used to observe WireGuard
 	// peer session state transitions.
 	//