Allow batch_size to work with joinable map tables and highwater

mikelutz → changed the visibility of the branch 3463800-allow-batchsize-to to hidden.

Thanks for working on this. I tested the MR and I got a never ending migration, processing the same records over and over. I think that is caused because reading the highwater value to add the condition to the query only happens in the first run of the batch inside an if ($this->batch == 0) condition.

The very first time that I run the migration, the highwater has not been set. So, the code from the MR keeps returning a query with LIMIT [BATCH_SIZE] OFFSET 0. Subsequent calls initializeIterator already have $this->batch greater than 0 so the code to add the highwater is never reached.

If I force the migration to stop and run it again, the highwater condition is set, but it is never updated in subsequent calls initializeIterator because of the same reason.

For this to work, I think the code that sets the highwater condition needs to be moved outside the if ($this->batch == 0) condition.