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_default.go

@@ -7,6 +7,7 @@ package derp
 
 import "context"
 
-func (c *sclient) statsLoop(ctx context.Context) error {
-	return nil
+func (c *sclient) startStatsLoop(ctx context.Context) {
+	// Nothing to do
+	return
 }