Returns nonzero if new request can be sent from local endpoint.

This function return 0 if request is not allowed for this session.
There are several reasons why request is not allowed.  Some of the
reasons are: session is server; stream ID has been spent; GOAWAY
has been sent or received.

The application can call `nghttp2_submit_request2()` without
consulting this function.  In that case,
`nghttp2_submit_request2()` may return error.  Or, request is
failed to sent, and :type:`nghttp2_on_stream_close_callback` is
called.