derp: remove stats goroutine, use a timer

Without changing behaviour, don't create a goroutine per connection that
sits and sleeps, but rather use a timer that wakes up and gathers
statistics on a regular basis.

Fixes #12127

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ibc486447e403070bdc3c2cd8ae340e7d02854f21

derp/derp_server.go

@@ -776,7 +776,6 @@ func (c *sclient) run(ctx context.Context) error {
 	var grp errgroup.Group
 	sendCtx, cancelSender := context.WithCancel(ctx)
 	grp.Go(func() error { return c.sendLoop(sendCtx) })
-	grp.Go(func() error { return c.statsLoop(sendCtx) })
 	defer func() {
 		cancelSender()
 		if err := grp.Wait(); err != nil && !c.s.isClosed() {
@@ -788,6 +787,8 @@ func (c *sclient) run(ctx context.Context) error {
 		}
 	}()
 
+	c.startStatsLoop(sendCtx)
+
 	for {
 		ft, fl, err := readFrameHeader(c.br)
 		c.debugLogf("read frame type %d len %d err %v", ft, fl, err)