Add an independent Sidekiq memory killer thread

What does this MR do?

Implement step 1) and step 2) in https://gitlab.com/gitlab-org/gitlab-ce/issues/59754#note_208104032

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Related to #59754 (moved)

Closes gitlab#27700 (closed)