The code inside process_topology_request_async has become too complex
therefore TopologyRequest has lost its utility

refactor: rename UpdateInstanceRequest -> update_instance::Request, UpdateInstanceResponse -> update_instance::Response

Use direct structure names instead of aliases for better IDE code surfing

Rewrite it from scratch to satisfy tests and to represent governor
logics in general.

Cases covered:
- when someone goes offline
- whenever vshard gets reconfigured

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.