Gonna extract this into its own method.

I considered integrating the sentinel into Einhorn's mourning process, but besides it complicating the entire upgrade process (which started out as fairly obscure already!), I don't see much of a benefit. /:

I think that with `upgrade` and `reload` being blocking operations to einhornsh (which is great!), it would be pretty unexpected if commands issued between issuing the `upgrade` command and finishing the upgrade would be served: e.g., what would incrementing the worker count pre-upgrade do?