[WIP][feature/alt-session-storage] Redis/Memcache storage for sessions

This branch replaces the pull #1568 with a completely different approach to how the backend sessions are written. This pull targets memcache and redis specifically, but because of the storage abstraction can be expanded up with later pull requests.

The reason for the change is the caching subsystem has various hooks and can be deleted at any time. Also, by specifically targeting these two storage systems, higher performance can be achieved by taking advantage of the special features of each.

PHPBB3-11705