Add grouped cache loader for caches sharing an invalidation domain

The cache loader added in XCOMMONS-3300 currently doesn't really support loading two different types of values - while it supports storing the loaded value wherever you want it, the load method assumes that all load calls for a key return the same value and the result can thus be returned to all callers. The cache loader should support a group of caches that load different types of values that share the same invalidation domain (keys). An example for this is the XWikiCacheStore that has both an exists and a page cache, both over pages as key, that need to be invalidated in a synchronized way.