To better communicate the contract for persistent cache factory:
if the cache.directory property is unset, or disk limit is zero or
negative, fall back to in-memory only caches
extract persistent cache base factory class that performs the check and
delegates to in-memory only cache factory or delegate to methods to
continue with persistent cache creation.
That would simplify the implementation for alternative persistent cache
backends.
Change-Id: Ieda138b1ff867c5d5fbbf3b67338e0f0b0a3aa95
3e656952f1