Make connection and stream windows configurable

[Tratcher]

Meta issue: #2627

Today the connection and stream window sizes are hardcoded at the default 65kb. These need to be independently configurable.

This maps to SETTINGS_INITIAL_WINDOW_SIZE.

[Tratcher]

Protocol default: 65k
Http.Sys/IIS default: 1mb (observed in handshake), and then it follows up immediately with a window update of 960KB (1mb - 65kb).
Apache default: 65kb

[halter73]

Is this for both the connection and stream windows?

[Tratcher]

The setting applies to both and the window update applies to the connection (no requests were sent in my test).

[Tratcher]

@halter73 have you started on this or should I steal it?

[halter73]

As discussed, by all means steal it. Thanks.