Client: On the first connection does a full Open and detects if the
server supports padding by checking for "Padding" header in the
response. Applies padding if the server does. In the following
connections it's back to Fast Open.
Server: Detects if the client supports padding by checking for "Padding"
header in the CONNECT request. Applies padding if the client does.
Both client and server always send "Padding" headers to somewhat protect
the request and response headers' packet lengths, even if the other side
may not acknowledge padding negotiation, either due to old version or
"Padding" headers being dropped by the frontend.
The manual option --padding is removed.
