Make Doctrine caching class more configurable.

Now caching class can be:

 * ApcCache
 * ArrayCache
 * Cache
 * CacheProvider
 * MemcacheCache
 * MemcachedCache
 * WinCacheCache
 * XcacheCache
 * ZendDataCache

to configure set:

```php
array(
  'doctrine.odm.mongodb.metadata_cache' => 'ApcCache',
),
```

with appropriate caching system do you require.