[18.10] Backport of 'Use primary DB connection for advanced search bulk indexer'

What does this MR do and why?

Backports !236265 (merged) to 18-10-stable-ee.

Fixes a race in ElasticIndexBulkCronWorker where it could read pre-commit data from a lagged replica and write the stale row to Elasticsearch, leaving the index stale until the next write to that record. The worker now uses the primary DB connection. See #600270 (closed) for details.

Cherry-pick of commit 68e66f2ede761868dff531da42fc86793bea8269 from master. Clean cherry-pick (no conflicts).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
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  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, reach out to a Software Engineer in Test in #s_developer_experience.

Note to the merge request author and maintainer

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• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.