We should only keep incoming closed streams because we only keep at
most max concurrent streams, which only applied to incoming streams.

Previously, stream object for pushed resource was not created during
nghttp2_submit_push_promise().  It was created just before
nghttp2_before_frame_send_callback was called for that PUSH_PROMISE
frame.  This means that application could not call
nghttp2_submit_response for the pushed resource before
nghttp2_before_frame_send_callback was called.  This could be solved
by callback chaining, but for web server with back pressure from
backend stream, it is a bit unnecessarily hard to use.

This commit changes nghttp2_submit_push_promise() behaviour so that
stream object is created during that call.  It makes application call
nghttp2_submit_response right after successful
nghttp2_submit_push_promise call.

See GH-436

Application may use it using nghttp2_stream_* functions, and traverse
its dependency.

Previously, nghttp2_session_end_request_headers_received assumes
stream is still writable (in other words, local endpoint has not sent
END_STREAM).  But this assumption is false, because application can
send response in nghttp2_on_begin_frame_callback.  Probably, this
assumption was made before the callback was introduced.  This commit
addresses this issue.  Since all
nghttp2_session_end_*_headers_received functions are identical, we
refactored them into one function.

Because of the nature of heap based priority queue, and our compare
function, streams with the same weight and same parent are handled in
the reverse order they pushed to the queue.  To allow stream pushed
earlier to be served first, secondary key "seq" is introduced to break
a tie.  "seq" is monotonically increased integer per parent stream,
and it is assigned to stream when it is pused to the queue, and gets
incremented.

Previously, nghttp2_session_find_stream(session, 0) returned NULL
despite the fact that documentation said that it should return root
stream.  Now it is corrected, and it returns root stream as
documented.

We added spdylay build since integration tests require it.