Send window size API extension
Motivation:

The send window size is currently fixed by a macro at compile time.
In order for users of the library to impact the send window size they
would have to change a macro at compile time. The window size may be dynamic
depending on the environment and deployment scheme. The library users
currently have no way to change this parameter.

Modifications:

Add a new optional callback method which is called before data is sent to
obtain the desired send window size. The callback return value will be
subject to a range check for the current session, stream, and settings
limits defined by flow control.

Result:
Library users have control over their send sizes.
Merge branch 'window_size_control' of https://github.com/akamai/nghttp2 into akamai-window_size_control
Merge branch 'akamai-window_size_control'