Chunk query that can potentially OOM

[aidvu]

Unbound query has potential to return millions of rows, and it's fairly easy to run into memory problems.

e.g. we had a problem with a ~4GB DB that was returning 13 million rows to check for the meta_value column.

The fix loops through the table fetching 1000 at a time. Added ORDER BY statement, so we can safely iterate through the primary keys.

Using ASC ordering, so the new items come last, in case something gets added while it's running.

[aidvu]

Review with hide whitespace changes. :)

[schlessera]

Thanks for the PR, @aidvu !

[aidvu]

Glad I could help.