derp: reduce excess goroutines blocking on broadcasts

Observed on one busy derp node, there were 600 goroutines blocked
writing to this channel, which represents not only more blocked routines
than we need, but also excess wake-ups downstream as the latent
goroutines writes represent no new work.

Updates #self

Signed-off-by: James Tucker <james@tailscale.com>

derp/derp_server.go

@@ -1573,6 +1573,17 @@ func (c *sclient) sendMeshUpdates() error {
 	c.s.mu.Lock()
 	defer c.s.mu.Unlock()
 
+	// allow all happened-before mesh update request goroutines to complete, if
+	// we don't finish the task we'll queue another below.
+drainUpdates:
+	for {
+		select {
+		case <-c.meshUpdate:
+		default:
+			break drainUpdates
+		}
+	}
+
 	writes := 0
 	for _, pcs := range c.peerStateChange {
 		if c.bw.Available() <= frameHeaderLen+keyLen {