ISPN-3290 Redesign CacheStore API

https://issues.jboss.org/browse/ISPN-3290
Notes:
- I reverted the change with the upgrade tool as merging it was a nightmare. I'll add it at a further point after the release together with tooling for compatibility with the old cache loader API (required for JDG)
- the new API is in this commit: https://github.com/mmarkus/infinispan/commit/d02eedb66c45297a26139570d6821c1c2d68f278
- the overall code still needs scrubbing but, given the importance of the change, I'd want to have it in rather soon in the release assuming the suite passes. Here's my remaining TODOs list for the task: https://gist.github.com/mmarkus/6449136


The following stores should also be integrated in lock-step, as are required by the server:
- https://github.com/infinispan/infinispan-cachestore-rest/pull/3 
- https://github.com/infinispan/infinispan-cachestore-leveldb/pull/4