Special treatment for Oracle

The generated sql statements for oracle (especially trigger statements) need some additional love to be executed consecutive in a script.

@butonic @LukasReschke @icewind1991 please review - THX
@butonic all true and we need to take care about this issue - one day(tm)

But in this case it's not of interest because for oracle we don't do this migration checking magic:

```php
class OracleMigrator extends NoCheckMigrator