[ticket/13421] Add an interface for \phpbb\db\tools

PHPBB3-13421
[ticket/13421] Inherit docs

PHPBB3-13421
[ticket/13421] Always require the interface when possible

PHPBB3-13421
[ticket/13421] Move tools to subdirectory

PHPBB3-13421
Merge pull request #3220 from nickvergessen/ticket/13421

Ticket/13421 Introduce an interface for db\tools.php