device: fix data race in peer.timersActive

Found by the race detector and existing tests.

To avoid introducing a lock into this hot path,
calculate and cache whether any peers exist.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

device/peer.go

@@ -125,6 +125,7 @@ func (device *Device) NewPeer(pk NoisePublicKey) (*Peer, error) {
 	// add
 
 	device.peers.keyMap[pk] = peer
+	device.peers.empty.Set(false)
 
 	// start peer