allow control over the inbound HTTP/2 flow control window

[weissi]

For best bulk transfer rates, it'd be great to control the HTTP/2 flow control window. Right now, I think we leave the defaults which are (IIRC) 64k per connection and 64k per stream. That means the server can't send a huge amount of data especially under a lot of concurrent streams. Also, we'll need to send a large number of WINDOW_UPDATE frames to up the flow control windows all the time.

Thank you to @Lukasa for reminding me that I want this when we were chatting about bulk H2 data transfer with AHC.