Remove debug output
Return new stream ID from nghttp2_submit_{request, headers, push_promise}

Previously stream ID was assigned just before HEADERS or PUSH_PROMISE
was serialized and nghttp2_submit_{request, headers, push_promise} did
not return stream ID.  The application has to check assigned stream ID
using before_frame_send_callback.  Now it is apparent that priority is
meant to DATA transfer only.  Also application can reorder the
requests if it wants. Therefore we can assign stream ID in
nghttp2_submit_* functions and return stream ID from them.  With this
change, now application does not have to check stream ID using
before_frame_send_callback and its code will be simplified.
Remove adjust_priority_callback

Since we have stream ID immediately available from nghttp2_submit_*,
we don't need adjust_priority_callback.
doc: Update tutorial-client.rst
nghttp: Remove streams from HttpClient