This is a patch for supporting toggling on/off
of foreign key checking in ndb.
Innodb supports

set foreign_key_checks = 0;

to immediately disable all foreign key checking, even
if set inside an ongoing transaction. All operations
prior to the setting will have foreign key constraints
checked and all operations after will not. For Innodb
these checks are done immediately, but for Ndb the setting

set ndb_deferred_constraints = 1;

will defer the foreign key checks until commit time.
The setting of foreign_key_checks = 0; inside a transaction
will only affect operations executed after the change,
operations done before will cause foreign keys to be checked.

The setting of foreign_key_checks = 0 will of course also
disable any defined cascade of deletes or updates.

The implementation disables Ndb's internal triggers for
foreign key checking inside TUP so it will also have a
performance improvement during restoring backups with
foreign key checks disabled.