use a monotonous timer for the connection (#3570)

There's no point in having the timer fire multiple times for the same
timestamp. By using a monotonuos timer we avoid busy-looping in cases where the
timer fires, but we can't actually send a packet.

internal/utils/timer.go

@@ -47,6 +47,10 @@ func (t *Timer) SetRead() {
 	t.read = true
 }
 
+func (t *Timer) Deadline() time.Time {
+	return t.deadline
+}
+
 // Stop stops the timer
 func (t *Timer) Stop() {
 	t.t.Stop()