Lower project export batch_size even further

What does this MR do?

This MR reduces export batch size even further from 20 to 2 in order to try and help users export project from GitLab.com that currently fails with db statement timeout (see #212355 (closed)). Original batch size reduction from 100 to 20 did not help so I am reducing it even further to see if that helps. Right now there is no visibility into which exact sql query fails so its hard to say what the underlying issue is. Additionally to this change I am adding sql query logging !34630 (closed) to get more insight.

Change is behind feature flag export_reduce_relation_batch_size which is only enabled for 1 user of GitLab.com

Mentions #212355 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team