derp: add short queues between reader and writer, drop on overload.

This avoids the server blocking on misbehaving or heavily contended
clients. We attempt to drop from the head of the queue to keep
overall queueing time lower.

Also:
 - fixes server->client keepalives, which weren't happening.
 - removes read rate-limiter, deferring instead to kernel-level
   global limiter/fair queuer.

Signed-off-by: David Anderson <dave@natulte.net>

cmd/derper/derper.go

@@ -40,7 +40,6 @@ var (
 	configPath    = flag.String("c", "", "config file path")
 	certDir       = flag.String("certdir", tsweb.DefaultCertDir("derper-certs"), "directory to store LetsEncrypt certs, if addr's port is :443")
 	hostname      = flag.String("hostname", "derp.tailscale.com", "LetsEncrypt host name, if addr's port is :443")
-	mbps          = flag.Int("mbps", 5, "Mbps (mebibit/s) per-client rate limit; 0 means unlimited")
 	logCollection = flag.String("logcollection", "", "If non-empty, logtail collection to log to")
 	runSTUN       = flag.Bool("stun", false, "also run a STUN server")
 )
@@ -120,9 +119,6 @@ func main() {
 
 	s := derp.NewServer(key.Private(cfg.PrivateKey), log.Printf)
 	s.WriteTimeout = 2 * time.Second
-	if *mbps != 0 {
-		s.BytesPerSecond = (*mbps << 20) / 8
-	}
 	expvar.Publish("derp", s.ExpVar())
 
 	// Create our own mux so we don't expose /debug/ stuff to the world.
@@ -196,7 +192,6 @@ func debugHandler(s *derp.Server) http.Handler {
 <ul>
 `)
 		f("<li><b>Hostname:</b> %v</li>\n", *hostname)
-		f("<li><b>Rate Limit:</b> %v Mbps</li>\n", *mbps)
 		f("<li><b>Uptime:</b> %v</li>\n", tsweb.Uptime())
 
 		f(`<li><a href="/debug/vars">/debug/vars</a> (Go)</li>