proposal: x/net/http2: add support for configuring stream and connection flow control values

[elindsey]

Today http2.Transport has hard-coded defaults for stream and connection flow control values. Stream flow control is determined by SETTINGS_INITIAL_WINDOW_SIZE sent on the initial SETTINGS frame, currently hardcoded to 4mb. Connection flow control has an RFC default of 65,535 bytes and is further increased by sending a WINDOW_UPDATE after the initial SETTINGS frame, which http2.Transport currently issues with a value of 1gb.

Both of these settings are important when it comes to controlling how the data from different streams is requested/interleaved. Exposing these in the Transport is in line with the parameters currently exposed on http2.Server, namely MaxUploadBufferPerConnection and MaxUploadBufferPerStream.

[ianlancetaylor]

CC @neild @bradfitz

[elindsey]

@neild any thoughts on adding these parameters? The defaults are high and allow for abuse depending on where/how Transport is exposed; I'm currently carrying out of tree patches to tweak these down.