- Support HA_EXTRA_ADD_CHILDREN_LIST,HA_EXTRA_ATTACH_CHILDREN,
  HA_EXTRA_IS_ATTACHED_CHILDREN and HA_EXTRA_DETACH_CHILDREN
  in partition handler for handlers that has HA_CAN_MULTISTEPL_MERGE flag
- Added HA_CAN_MULTISTEPL_MERGE to MERGE handler.
- Added handler::get_child_handlers()
- Change m_num_lock to contain total number of locks. This was needed as we
  now adjust number of locks when extra(HA_EXTRA_ATTACH_CHILDREN) is called.