FULL OUTER JOIN Allowed JOIN Orders

Taking from MDEV-36813:

If the query has

  t1 full outer join t2 on on_cond

then t1 and t2 may only be together in the join order.
It can be ... t1 t2 ... or ... t2 t1...

Need to modify check_interleaving_with_nj() and JOIN::get_allowed_nj_tables()
to enforce that.

(Just have each table remember its full-outer-join peer, if any. If the last table
in the join order has a peer, only allow that peer as the next table)