device: get rid of peers.empty boolean in timersActive

There's no way for len(peers)==0 when a current peer has
isRunning==false.

This requires some struct reshuffling so that the uint64 pointer is
aligned.

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

device/device.go

@@ -55,9 +55,13 @@ type Device struct {
 		publicKey  NoisePublicKey
 	}
 
+	rate struct {
+		underLoadUntil int64
+		limiter        ratelimiter.Ratelimiter
+	}
+
 	peers struct {
-		empty        AtomicBool // empty reports whether len(keyMap) == 0
-		sync.RWMutex            // protects keyMap
+		sync.RWMutex // protects keyMap
 		keyMap       map[NoisePublicKey]*Peer
 	}
 
@@ -65,11 +69,6 @@ type Device struct {
 	indexTable    IndexTable
 	cookieChecker CookieChecker
 
-	rate struct {
-		underLoadUntil int64
-		limiter        ratelimiter.Ratelimiter
-	}
-
 	pool struct {
 		messageBuffers   *WaitPool
 		inboundElements  *WaitPool
@@ -135,7 +134,6 @@ func removePeerLocked(device *Device, peer *Peer, key NoisePublicKey) {
 
 	// remove from peer map
 	delete(device.peers.keyMap, key)
-	device.peers.empty.Set(len(device.peers.keyMap) == 0)
 }
 
 // changeState attempts to change the device state to match want.