This commit makes handling of outgoing HEADERS and PUSH_PROMISE in the
same priority of other frames on the stream, so these frames are
processed in the order they are submitted.  This allows application to
submit frames to a stream returned by nghttp2_submit_{request,
headers, push_promise} immediately.  The only exception is
WINDOW_UPDATA frame, which requires nghttp2_stream object, which is
not created yet.