Fix launch errors, improve Pods API, use hook abstraction

Sorry for the largeness of this PR, just took a while to get it all together. @mpuncel @petertseng 

This extracts the preparer's orchestration logic to a package that can be tested. It does not solve the self-hosting issue yet, but gets us most of the way there.
do we still need the retries then? if we get no supervise/ok as the error we don't really need to wait around to see if it goes away do we? (I don't know the answers to those questions)