device: use channel close to shut down and drain outbound channel

This is a similar treatment to the handling of the encryption
channel found a few commits ago: Use the closing of the channel
to manage goroutine lifetime and shutdown.
It is considerably simpler because there is only a single writer.

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

device/peer.go

@@ -17,7 +17,7 @@ import (
 )
 
 const (
-	PeerRoutineNumber = 3
+	PeerRoutineNumber = 2
 )
 
 type Peer struct {
@@ -287,7 +287,6 @@ func (peer *Peer) Stop() {
 
 	peer.queue.Lock()
 	close(peer.queue.nonce)
-	close(peer.queue.outbound)
 	close(peer.queue.inbound)
 	peer.queue.Unlock()