Improve performance of doGetStats

Hi,

by calling apc_cache_info without the limited parameter set, we are actually fetching ALL entries that are currently cached in APC. (http://php.net/manual/en/function.apc-cache-info.php)
As the code bellow does not need these entries for the statistics we should set the limited parameter to true to avoid performance issues if a large amount of APC-Cache has already been gathered.
Done in doctrine/cache@85ff6118558d7b2501cd85b31bdc1ffc0cab731a