Async Migrator should have a default sync migration LIMIT

[fisehara]

To reduce the complexity of an async migration file <>.async.ts which currently needs to specify two runnable migration functions:

• asyncFn (runs with a limit to batch the affected rows)
• syncFn runs without a limit to assure data consitency.

The default approach should be, that the async migrator executes the syncFn if specified.
If it's not specified it will run the asyncFn with a LIMIT ALL which is the same as not giving a limit.
This is a postgres specific case and a mysql case needs to be specified.

Then finalizing an async migration will automatically run the asyncFn without a limit.