The sql_save function cannot take arguments by reference since it is called
by call_user_func_array()
Replace use of isset($cache->sql_rowset[$query_id]) with $cache->sql_exists
Replace $cache->cache_dir with $cache->get_driver()->cache_dir
PHPBB3-10875
They are now refered to as cache drivers rather than ACM classes. The
additional utility functions from the original cache class have been
moved to the cache_service. The class loader is now instantiated without
a cache instance and passed one as soon as it is constructed to allow
autoloading the cache classes.
PHPBB3-9983
