As suggested in GH-674

This commit also fixes the bug that SETTINGS timer continues after
connection was closed.

This script prints out the commit author from `git log` output.  Used
to update AUTHORS file.

2 APIs are added.  nghttp2_session_get_local_window_size() returns the
amount of data that the remote endpoint can send without receiving
connection level WINDOW_UPDATE.
nghttp2_session_get_stream_local_window_size() returns the amount of
data that the remote endpoint can send without receiving stream level
WINDOW_UPDATE.

There is a discussion in httpbis mailing list which argues that
dependency tree is for client, and changing it in server side is not
what client expects.
https://lists.w3.org/Archives/Public/ietf-http-wg/2016JulSep/0416.html

Currently, we make pushed stream depend on the parent stream of
associated stream (that is main HTML in most of the cases), so that
associated stream and pushed stream become siblings.  In this case, we
also observed that these resources complete each other to get its
parent weight.  This means that the delivery of associated stream is
delayed by pushed streams.

So at this moment, it is not a good idea to change pushed stream
priority in a way we do currently.