Using 'sequence' generator in database mapping causes unexpected COMMIT in transaction

### Steps to Reproduce
Let's have a domain using
```
class MyDomain {
...
static mapping = {
id generator: 'sequence'
}
...
}
```

Since generation of each id causes commit in current transaction, following code behaves unexpectedly:
```
MyDomain.withTransaction { status ->
def d1 = new MyDomain(someProperty: 'Foo')
d1.save(flush: true)

def d2 = new MyDomain(someProperty: 'Bar')
d2.save(flush: true) <-- sequence id generator calls COMMIT, that commits also d1

status.setRollbackOnly() <-- just d2 is rolled back
}
```

### Expected Behaviour
Both domains d1 and d2 should be rolled back at the end of the transaction.

### Actual Behaviour
**MySQL**:
Since we use MySQL, hibernate uses org.hibernate.id.enhanced.TableGenerator as fallback. As soon as this sequence generator updates and commits next_val in hibernate_sequences table, the MyDomain d1 is also commited during generating of the id of d2, so the rollback at the end of the transaction rolles back just d2.

**Postgres**:
We also tried to use Postgres database, where no hibernate_sequences table has been created. The unexpected commit is performed same as in MySQL case.

**H2**:
It works correctly when H2 is used.

Remark:
We debugged the code also with our own generator derived from org.hibernate.id.enhanced.TableGenerator calling just super.generate() method and it seems as if the generator doesn't use new connection while updating next_val in hibernate_sequences table.

### Environment Information
- **Operating System**: Windows 10
- **Grails Version:** 3.3.9 (tested also in also 2.5.6)
- **JDK Version:** TODO
- **Container Version (If Applicable):** Tomcat bundled with Grails
- **MySQL, Postgres**

### Example Application
https://github.com/jhron/GrailsSequenceTest
Just update dataSource settings in application.yml and bootRun. Then call http://localhost:8080/person/create