The Ndb foreign key implementation does not correctly support
cascading delete where the child table has Blob/Text columns.

Therefore we should not permit users to create foreign keys
of this sort.

A new Ndb error is added :
  21034 Create foreign key failed - child table has Blob or Text column and
on-delete-cascade is not allowed

The ndb_fk_cascade_delete.test testcase is extended to check
that such foreign keys can no longer be created.

Reviewed-by : Martin Skold <martin.skold@oracle.com>