ISPN-3318 Embedded rolling upgrade using CLI

* Create a brand new read-only cache loader that uses CLI to
retrieve data from other caches.
* A few adjustments have been made to the CLI code to accomodate
this new cache loader.
* Providing rolling upgrade capabilities for embedded caches is the
key step for providing on-the-fly cache store data migration.
* Data stored in old formats in embedded Infinispan 5.2.x instances
can easily be migrated to Infinispan 6.x embedded instances plugged
with different cache stores, or cache stores with updated storage
format.
* Support for custom value clases but not custom class keys.