device: get rid of nonce routine

This moves to a simple queue with no routine processing it, to reduce
scheduler pressure.

This splits latency in half!

benchmark                  old ns/op     new ns/op     delta
BenchmarkThroughput-16     2394          2364          -1.25%
BenchmarkLatency-16        259652        120810        -53.47%

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

device/timers.go

@@ -87,7 +87,7 @@ func expiredRetransmitHandshake(peer *Peer) {
 		/* We drop all packets without a keypair and don't try again,
 		 * if we try unsuccessfully for too long to make a handshake.
 		 */
-		peer.FlushNonceQueue()
+		peer.FlushStagedPackets()
 
 		/* We set a timer for destroying any residue that might be left
 		 * of a partial exchange.