Updates the database with with any changes made by any methods that can update the field, which are: MEOW_Field::val, MEOW_Field::value, MEOW_Field::prop, __set. Note that because of the way MasterPress stores values, this method cascades up to the enclosing field set collection to do all of the updates in that collection at once, so if you need to make changes to multiple fields, you should call this method once after all changes have been made.