Add multiple events in \phpbb\session.php

Add core.update_session event to update_session function in \phpbb\session class.
Add core.session_create event - return session information that should be inserted in to the db to extension
Add core.session_kill event - returns the session that should be killed to extension.
Add core.session_gc event - Triger on garbige sessiona garbige collection.

Event should return $session_data and $session_id, but for security reasons should not allow changing of this information.

Reason - I have security extension I need to migrate from 3.0.x that kept track of user actions and gave approximate information for multi accounting.