Log Message:
Update php-apcu to 5.1.5

Changes:
- PHP 7 compatibility
- provide APC compatibility in "apc" optional extension
- move APCIterator to APC compatibility extension
- add APCuIterator without cache argument
- drop apcu_bin_* functions
- add apcu_entry(key, callback, ttl) function
- fix race on refcount of entry when using rwlocks (or on windows)

NOTE: apc-compatible interfaces moved into separate module