Increase batch size when streaming client audit logs

[matthew-white]

The client_audits table can get pretty big. One form that I've encountered had 16K submissions, but 1.2 million client_audits rows. It took 2 minutes to download the submissions.csv.zip for this form, most of which was spent streaming from client_audits (via ClientAudits.streamForExport()). The resulting submissions.csv.zip was 15 MB, and the combined client audit log CSV file was 105 MB unzipped.

It'd be nice to improve the performance of this streaming. It seems possible that a form could have even more client_audits rows. One idea I have for improving the performance is increasing the batch size when streaming from Postgres. Configuring the batch size isn't documented in the Slonik readme, but there was a PR for it here: gajus/slonik#322. However, the PR was released with 27.1.0, which is ahead of the version of Slonik that we're on.

[matthew-white]

Increasing the batch size is blocked by #1119.