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.
Move persistence sources into a deeper bundle in preparation for
splitting of persistence into two bundles - one specific to the
Couch adapter, and one for persistence caching generally. This
supports WTD-702, the WARP persistence adapter, which will not
use Couch for persistence but which will want to use the platform-
provided persistence cache.
