Tune zero downtime reindexing using test framework

Background

Zero downtime reindexing is not stable with large datasets and we have no way to test different configurations out.

Proposal

Use the production data dumps to grab a copy of gitlab-org and load that into a a test cluster.

Use existing benchmark rally tracks and test out different zero downtime reindexing settings to find the optimal settings for reindexing

Those settings should be used when using zero downtime reindexing on GitLab.com to validate performance/impact