Fixes#7527
I want to highlight this and explain what I think the implications are and make sure we are aware:
* `HTTPConnStateFunc` closes the connection when it is beyond the limit. `Close` does not block.
* `HTTPConnStateFuncWithDefault429Handler(10 * time.Millisecond)` blocks until the following is done (worst case):
1) `conn.SetDeadline(10*time.Millisecond)` so that
2) `conn.Write(429error)` is guaranteed to timeout after 10ms, so that the http 429 can be written and
3) `conn.Close` can happen
The implication of this change is that accepting any new connection is worst case delayed by 10ms. But only after a client reached the limit already.
