Immutable CachedFiles

Previously, the CachedFile always behaved as if it was mutable, even
in situations where it was working against an immutable cache.  This
resulted in situations where a combination of intermittent network
connections and read-only file systems would cause an attempted
direction creation that would fail.  This change enhances the
CacheFile such that it can behave in both a mutable and immutable
fashion.

[#71928434][#54]
Merge 71928434-mkdir-immutable-cache to master

[Completes #71928434]