Split out adding of individual request headers into a switch. Check
the connection http version only on fresh connections, use separate
methods.
Add TE: header directly without allocation. Add bit for indicating
Connection: header has been added and custom headers should not do
that again.
Closes#18444
Set the initial stream window size to 64KB and increase that to the 10MB
we used to start with on the first server reply, unless a rate limit is
in effect.
Continously monitory changes to the transfers rate limit and adjust the
stream window size accordingly. `max_recv_speed` is a transfer propert
that can be changed during processing by a callback.
Closes#14326
