Shutdown service cleanly

[nuke-web3]

If we can, we want to stop the service in a way that leaves the DB in a good state.

See:

• https://docs.rs/tokio-util/latest/tokio_util/sync/struct.CancellationToken.html
• https://docs.rs/tonic/latest/tonic/transport/server/struct.Router.html#method.serve_with_shutdown

want to use a tokio::select! to decide how to shutdown & notify processes (sled, tonic) and do whatever token comes first

[nuke-web3]

#43