OGM-617 "Cache per table" strategy for Ehcache

@emmanuelbernard, @alexsnaps: Could you check this one out?

Alex, this is the new persistence strategy I mentioned yesterday. By default, we now use a specific cache per table. One can go back to the other strategy via a configuration option. Caches can be configured indvidually, as a fallback the configuration from ENTITIES, ASSOCIATIONS and IDENTIFIERS are used as a basis.
Well, the _right way_ may be a bit of an overstatement, but the best choice you have yes...