Allow worker to accept a filesystem store and update basic_cas

1. Change the logic in the worker to allow it to accept a filesystem store as well then change the basic_cas.json accordingly.

The problem is that in the basic_cas.json example we only use a fast store and the slow store is sent to a noop (/dev/null store). Because this change made fast_slow store always bypass the fast store when checking if an item exists it ends up always getting a 404 not found error from the noop store.

We require a fast_slow store for a store that is wired up to a worker, since production systems would have the CAS and workers live in separate instances (at least separate processes), we simplified the code a lot by recycling the fast_slow store logic to ensure things are placed into a filesystem store part of the fast store.

Extension of #665

Read the worker's store configuration logic and basic_cas.json, then compare the requested behavior with extension #665. Confirm the worker accepts a filesystem store and update basic_cas.json so the fast_slow setup no longer routes existence checks to the noop store; done when the example represents the intended filesystem-backed arrangement.