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.

This change is for the future enhancement where we loose the
requirement about the chunk size for each buffer.

to simplify logic

Add nghttp2_on_invalid_header_callback

return value check in nghttp2_hd_inflate_hd_nv().

Merge from nghttp2/nghttp2