Add database capture thread

Jon Jenkinsrequested to merge
capture-poc into master

What does this MR do and why?

This is a continuation of the work started in !197240 (closed)

Related to #548606

How to set up and validate locally

  1. Set the database_capture feature flag to 'true'
  2. Start gitlab
  3. Verify the capture process has started (see referenced MR)
  4. Perform some actions that run queries
  5. Verify the existence of the captured data in /tmp

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