Move cache sources into their own bundle, for reuse with other
persistence adapters; specifically supports the persistence
adapter to the WARP Server, which is non-Couch but which will
want to use this cache. WTD-702.
Update README files to document the reorganization of persistence
sources into separate bundles for the Couch adapter and for the
persistence cache, in support of reusing the latter with a
different persistence adapter, WTD-702.
