device: remove starting waitgroups

In each case, the starting waitgroup did nothing but ensure
that the goroutine has launched.

Nothing downstream depends on the order in which goroutines launch,
and if the Go runtime scheduler is so broken that goroutines
don't get launched reasonably promptly, we have much deeper problems.

Given all that, simplify the code.

Passed a race-enabled stress test 25,000 times without failure.

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

device/receive.go

@@ -111,7 +111,6 @@ func (device *Device) RoutineReceiveIncoming(IP int, bind conn.Bind) {
 	}()
 
 	logDebug.Println("Routine: receive incoming IPv" + strconv.Itoa(IP) + " - started")
-	device.net.starting.Done()
 
 	// receive datagrams until conn is closed
 
@@ -246,7 +245,6 @@ func (device *Device) RoutineDecryption() {
 		device.state.stopping.Done()
 	}()
 	logDebug.Println("Routine: decryption worker - started")
-	device.state.starting.Done()
 
 	for {
 		select {
@@ -321,7 +319,6 @@ func (device *Device) RoutineHandshake() {
 	}()
 
 	logDebug.Println("Routine: handshake worker - started")
-	device.state.starting.Done()
 
 	for {
 		if elem.buffer != nil {
@@ -521,8 +518,6 @@ func (peer *Peer) RoutineSequentialReceiver() {
 
 	logDebug.Println(peer, "- Routine: sequential receiver - started")
 
-	peer.routines.starting.Done()
-
 	for {
 		if elem != nil {
 			if !elem.IsDropped() {