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.

This avoid establishing HTTP/2 backend connection again w/o pending
request.

See GH-431

The added API is nghttp2_session_change_stream_priority().  This
provides the same functionality to re-prioritize stream when PRIORITY
frame.  is received, but we do it without PRIORITY frame.  This could
be useful for server to change pushed stream's priority silently.

This commit adds ability to check status of cached file descriptor to
make sure that it can be reused.  We inspect last modification time
and number of hard links.  If last modification is changed from the
last validation time, or number of hard links gets 0, we don't reuse
file descriptor.  We also capped upper limit of the cached file
descriptors.  If the limit is reached, we will close file descriptor
which is least recently used, and its usecount is 0.

This is very simple API, and it returns nonzero if session is
initialized as server.