Add mechanism to know if a bundle was successfully loaded or not

[jappeace]

Currently at supercede we deploy our app trough nixops with a lot of custom nix logic[1],
however since keter always gaurantees an app is running, there is no way to know if a newly placed bundle in the incoming directory has succeeded or failed.
What I'd like in this issue is some kindoff interface, in the broadest sense of the word, to know if the bundle loaded or not.

What I'm doing at the moment as a work around is emit a log message in the app just before binding to a port. This message then get's detected by a systemd script which waits for that. This approach has a lot of false negatives (eg, it reports failure wheras it actually succeeded).

[1]: I intend to upstream this code as soon as I get to it. It's not that much tbh.

[jappeace]

maybe a hash of the tar + a timestamp of loading would achieve this (for the temp working dir, or add it to that cli thing, or both?)