GORM doesn't create referential integrity for joinTable

The auto-generated data model from grails / gorm creates referential integrity constraints... good.

However when you use the joinTable: clause, it doesn't create constraints.

An example of this is the grails async mail plugin:
https://github.com/kefirfromperm/grails-asynchronous-mail

This creates the following tables:
ASYNC_MAIL_MESS
ASYNC_MAIL_ATTACHMENT
ASYNC_MAIL_CC
ASYNC_MAIL_BCC
ASYNC_MAIL_TO
ASYNC_MAIL_HEADER

Only ASYNC_MAIL_ATTACHMENT has referential integrity with regards to ASYNC_MAIL_MESS, and that's because it uses a separate class for AsynchronousMailAttachment rather than the joinTable construct. All the other tables have no integrity.

By implication, options like cascade: 'all-delete-orphan' and so forth should also be allowed.

### Task List

Run the async mail plugin
Observe the auto-generated tables as listed above have no referential integrity.

### Expected Behaviour

I see no reason that joinTable shouldn't generate integrity like the rest of gorm.

### Actual Behaviour

No referential integrity.

### Environment Information

- **Grails Version:** 4.0.9
- **JDK Version:** 1.8

### Example Application

https://github.com/kefirfromperm/grails-asynchronous-mail

Run the linked grails-asynchronous-mail example with Grails 4.0.9 and inspect the generated ASYNC_MAIL_MESS, ASYNC_MAIL_ATTACHMENT, ASYNC_MAIL_CC, ASYNC_MAIL_BCC, ASYNC_MAIL_TO, and ASYNC_MAIL_HEADER tables. Compare the joinTable-generated schema with ASYNC_MAIL_ATTACHMENT, then verify that the relevant join tables have referential integrity and support the stated cascade behavior.

Written by the indexing model from the issue text.