fix: race condition in on_shutdown

Tarantool remote requests are handled in a IProto thread, and
on_shutdown triggers are handled in the TX thread.

During shutdown we send a request to the leader, which introduces a race
condition in case we are the leader. If iproto thread shuts down before
the request is sent, it will never be handled.

The fix is simple: in case we are the leader during shutdown, don't send
the request, instead handle the request directly in the TX thread.