[feature/alt-session-storage] Move common Redis methods into abstract class

PHPBB3-11705
[feature/alt-session-storage] Redis get_user_sessions should get more than 1 session

PHPBB3-11705
[feature/alt-session-storage] Basic memcache implementation

PHPBB3-11705
[feature/alt-session-storage] Fix memcache errors on empty keys

PHPBB3-11705
[feature/alt-session-storage] Remove unnecessary time limiter

PHPBB3-11705
Merge branch

Conflicts:
	phpBB/phpbb/session/storage/redis.php