MDEV-16465 Invalid (old?) table or database name or hang in ha_innobase::delete_table and log semaphore wait upon concurrent DDL with foreign keys

ALTER TABLE locks the table with TL_READ_NO_INSERT, to prevent the
source table modifications while it's being copied. But there's an
indirect way of modifying a table, via cascade FK actions.

After previous commits, an attempt to modify an FK parent table
will cause FK children to be prelocked, so the table-being-altered
cannot be modified by a cascade FK action, because ALTER holds a
lock and prelocking will wait.

But if a new FK is being added by this very ALTER, then the target
table is not locked yet (it's a temporary table). So, we have to
lock FK parents explicitly.