Follow up on SQLAbstract issue 4

[laurentszyster]

Following up on unframed/SQLAbstract.php#4, JSONModel must generate SQL constraints.

First the PRIMARY KEY constraint.

Then FOREIGN KEY constraints.

[laurentszyster]

Note that MySQL default storage ignores FOREIGN KEY constraints altogether.

To let MySQL applications implement referential integrity themselves, the delete method should support a cascade option that maps the related table's references.