This commit includes backward incompatible change, since we change
private field in public API class.

Previously we scheduled the transmission of response HEADERS using
priority tree in the belief that it allows more better utilization of
bandwidth for prioritized streams.  But to reduce the overhead of
reconstruction of priority queue when connection level flow control
window is depleted, we just don't check priority tree in this case.
This means that response HEADERS frames are not sent even though they
are not flow controlled.  This could waste bandwidth.  To improve this
situation, we stop scheduling response HEADERS with priority tree for
now.  Now they are just sent in the order they submitted.  The
response body DATA continued to be scheduled with priority tree as
before.

return session_inflate_handle_invalid_stream(...) case is for streams
for INITIAL state, but this is rare case.  In general, we'd like to
reduce RST_STREAM transmission, and it is suffice to ignore this frame
for now.

https://datatracker.ietf.org/doc/draft-ietf-httpbis-legally-restricted-status/

This makes h2spec strict mode a bit happier.  We still one failing
test with h2spec -S (strict mode).

This is a short hand for --disable-app --disable-examples
--disable-hpack-tools --disable-python-bindings, for users who want to
build libnghttp2 only.

To allow the asio wrapper to work with boost.fiber it is required
to access the underlying io_service objects.