Add cooperative file locking for cached certificate chain

[connorkuehl]

A more loquacious explanation is in the commit log.

FYI: requires a git dependency on this: https://github.com/connorkuehl/sev-cache

If this is a viable solution I'd like to move this repo to the enarx org.

[haraldh]

Do you need to sync with time consistency? Otherwise I find rename atomically enough.

[haraldh]

Or can we just make this a system requirement like /etc/hostname or hardware specific machine requirement. So /etc/sev-cert-chain or something

[connorkuehl]

Implementing a solution that doesn't require file locking.