ISPN-4409 Upgrade to LevelDB 0.7 and fix server test assertions

* Upgrade to LevelDB 0.7 in order to align with server's Guava
version which currently is 16.x.
* Fixed LevelDB server test assertions to avoid having to instantiate
the cache's marshaller client side.
ISPN-4012 Mark mass indexing tests as unstable
ISPN-2956 Hot Rod conditional operations and transactional caches

* Transactional caches are required in order to be able to guarantee
correct retry logic for Hot Rod operations relying on conditional
operations, or operations where the previous value is returned.
* WARN messages have been added for when conditional or operations
needing previous value are executed against non-transactional caches.