[feature/session-storage-cache] Added interfaces for cache backend

PHPBB3-11705
[feature/session-storage-cache] Move files to match new directory structure

PHPBB3-11705
[feature/session-storage-cache] Rename interface files to match new autoloading system

PHPBB3-11705
[feature/session-storage-cache] Move cache interface to match new directory structure

PHPBB3-11705
[feature/session-storage-cache] Changed interface, added Redis implementation, test.

PHPBB3-11705
[feature/session-storage-cache] Added test skip for platforms without fork

PHPBB3-11705
[feature/session-storage-cache] Changes to Redis atomic test for consistency

PHPBB3-11705
[feature/session-storage-cache] Added memcache implementation, test.

PHPBB3-11705
[feature/session-storage-cache] Combined atomic tests together

PHPBB3-11705
[feature/session-storage-cache] Added a start time which eliminates random failures

PHPBB3-11705
[feature/session-storage-cache] Typo, skip test if memcache is not loaded

PHPBB3-11705
[feature/session-storage-cache] Minor code changes for clarity

PHPBB3-11705