relational db: set all foreign key constraints to "initially deferred"

[r-peschke]

@m-schieder @reicda @rrenkert Since we can't guarantee the sequence of create events/write requests in current development, we have to set all foreign key constraints deferred. Simple example:

• agenda_item/1 with a content_object_id motion/5 should be inserted into db

• There exist following foreign key constraint: ALTER TABLE agenda_item_t ADD FOREIGN KEY(content_object_id_motion_id) REFERENCES motion_t(id);

  There is no problem inserting the motion/5 first and afterwards the agenda_item/1. The opposite is not true: Inserting the agenda_item/1 with the field content_object_id motion/5 first, throws ForeignKeyViolation: insert or update on table "agenda_item_t" violates foreign key constraint "agenda_item_t_content_object_id_motion_id_fkey", because the constraint is checked immediately. To move the check to the end of transaction we will change all foreign key constraints to the form ALTER TABLE agenda_item_t ADD FOREIGN KEY(content_object_id_motion_id) REFERENCES motion_t(id) INITIALLY DEFERRED;

Solving this issue we will write a new issue to revert this and try to remove the INITIALLY DEFERRED, because we expect it to be a time and/or memory penalty