This is pathological case, but we may get RST_STREAM to the promised
stream while we are sending PUSH_PROMISE. To handle this case,
instantiate Request object before transmission.

The option syntax is <PATH>=<PUSH_PATH,...>.
Push resources PUSH_PATHs when PATH is requested. This option can be used
repeatedly to specify multiple push configurations. For example,
  -p/=/foo.png -p/doc=/bar.css
PATH and PUSH_PATHs are relative to document root.

The nghttp2_submit_{request,response}2 functions are renamed as
nghttp2_submit_{request, response}.

cookie and set-cookie are treated specially and won't be concatenated.

This mechanism existed but was deleted. We bring it back in order to
prevent lower priority streams from starving.

Previously we use largest stream ID received so far as last-stream-ID,
and it is irrevant that it is passed to the callback (thus upper layer).
Now the stream ID which is passed to callback is eligible to
last-stream-ID.

This commit also changes SPDY's flow control size. Previously,
the size for SPDY is the same amount of bytes with HTTP/2.
For example, --frontend-http2-upstream-window-bits=N,
the window size is 2**N - 1. Now SPDY code uses 2**N.