[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