[2.0][DC-460] Refactored cache bulk deletion methods to use driver specific features to retrieve list of keys. Also, refactored tests
so that all methods are tested for all drivers.

Removed:

- Doctrine\Common\Cache\AbstractCache::count()
- Doctrine\Common\Cache\AbstractCache::deleteAll()

API Changes:

- Doctrine\ORM\AbstractQuery::getResultCacheId() now public

Bugs fixed:

- Doctrine\Common\Cache\AbstractCache::deleteByPrefix() was deleting _every_ key in cache

[2.0][DDC-171] Fixed (second attempt). Some other cleanups. Performance fix for mock Statement used in hydration performance tests for more accurate and better results.

[2.0] Fixed array tests by removing tests for ArrayCache specific methods from the tests of other cache drivers. Some general API work on the cache interface.