This is a better way to check errors, both semantically (no usually nil is
bad, but nil error is good, so this avoids a mental negation) and for
the reporting by the assert package.
Flamenco v2 allowed separate configuration of the Shaman file store and
checkout paths. This is now just one setting for "the storage". The file
store will be in `{storage}/file-store` and the checkout will happen in
`{storage}/jobs`.
Only add a random suffix to the checkout dir if it is necessary to ensure
uniqueness. If the client-supplied checkout directory doesn't exist yet,
it will be used as-is.
This introduces some more conceptual changes to Shaman. The most important
one is that there is no longer a "checkout ID", but a "checkout path".
The Shaman client can request any subpath of the checkout directory,
so that it can handle things like project- or scene-specific prefixes.
This is not yet working, it's just a direct copy of the Manager of Flamenco
2, with Logrus replaced by Zerolog. The API has been documented in
flamenco-manager.yaml as a starting point for the integration.
