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

PHPBB3-9733
[feature/session-storage] Documentation changes for code guidelines consistency

PHPBB3-9733
[feature/session-storage] Rename/Move files for autoloading

PHPBB3-9733
[feature/session-storage] Change phpdoc alignment

PHPBB3-9733
[feature/session-storage] Move set_db to native implementation

PHPBB3-9733
[feature/session-storage] Split storage interface into many interfaces

PHPBB3-9733
Merge branch 'feature/session-storage' into feature/session-storage-cache
[feature/session-storage] Move two user functions from storage -> user

PHPBB3-9733
Merge branch 'feature/session-storage' into feature/session-storage-cache
[feature/session-storage-cache] Merge storage cache classes & move for constancy

PHPBB3-11705
[feature/session-storage-cache] atomic_interface cache now extends cache_interface

PHPBB3-11705
[feature/session-storage-cache] WIP implementation of cache storage driver

PHPBB3-11705