MDL-41106 cache: several fixes for the session cache.
This issue makes several fixes for the session loader and the session store.
* maxsize argument now works for session caches.
* fixed performance hole when interation occurs frequently.
* fixed cache purge bug occuring when multiple caches are defined before being used.
* improved lastaccess handling.
Big thanks to Marina who contributed the following commits:
* Always make sure the elements in cache are sorted so we need to remove only elements in the beginning of array
* Remove expired elements from session store to free memory
* Minor bug fixes
