support for ON DELETE CASCADE

Both Hibernate and (I believe) EclipseLink have at least some sort of support for using `on delete cascade` to remove child entities when a parent entity is `remove()`d.

This is of course much more efficient than executing (potentially multiple) `DELETE` statements, with the downside that we can't promise that lifecycle callbacks get processed. But this isn't the only scenario where we skip lifecycle callbacks, and anyway this would be an opt-in thing.

So this is a candidate for standardization.

I guess the _simplest_ API for this would be to add a `onDelete` member to the `ForeignKey` annotation. The downside of this approach is it's a bit ugly:

```java
@ManyToOne
@JoinColumn(foreignKey=@ForeignKey(onDelete=CASCADE))
public Parent parent;
```

It also feels a bit unnatural to me to put this on the `@ManyToOne` side of the association instead of on the `@OneToMany` side where you would put `cascade=REMOVE`.

But I guess this is bearable.

[Hibernate has something slightly more natural, an `@OnDelete` annotation which may be specified at either end of the association, but it does feel like this rightly belongs in `@ForeignKey`.]

Start with the existing ForeignKey annotation and review how Hibernate and EclipseLink support ON DELETE CASCADE. The issue does not name files or tests; completion would require an agreed standard API, documented lifecycle-callback behavior, and corresponding implementation and tests.

Written by the indexing model from the issue text.