Signals to the client that the server started graceful shutdown
procedure.

This function is only usable for server.  If this function is
called with client side session, this function returns
:enum:`nghttp2_error.NGHTTP2_ERR_INVALID_STATE`.

To gracefully shutdown HTTP/2 session, server should call this
function to send GOAWAY with last_stream_id (1u << 31) - 1.  And
after some delay (e.g., 1 RTT), send another GOAWAY with the stream
ID that the server has some processing using
`nghttp2_submit_goaway()`.  See also
`nghttp2_session_get_last_proc_stream_id()`.

Unlike `nghttp2_submit_goaway()`, this function just sends GOAWAY
and does nothing more.  This is a mere indication to the client
that session shutdown is imminent.  The application should call
`nghttp2_submit_goaway()` with appropriate last_stream_id after
this call.

If one or more GOAWAY frame have been already sent by either
`nghttp2_submit_goaway()` or `nghttp2_session_terminate_session()`,
this function has no effect.

This function returns 0 if it succeeds, or one of the following
negative error codes:

:enum:`nghttp2_error.NGHTTP2_ERR_NOMEM`
    Out of memory.
:enum:`nghttp2_error.NGHTTP2_ERR_INVALID_STATE`
    The *session* is initialized as client.