[ticket/11601] Split post_setup_synchronisation logic from xml parsing

PHPBB3-11601
[ticket/11601] Add protected method for database sync and call it

PHPBB3-11601
Merge remote-tracking branch 'nickvergessen/ticket/11601' into develop-olympus

* nickvergessen/ticket/11601:
  [ticket/11601] Add protected method for database sync and call it
  [ticket/11601] Split post_setup_synchronisation logic from xml parsing