Implement support for specifying data consistency per database in Sidekiq workers

This issue tracks the effort to implement and roll out per-db data consistency in Sidekiq workers.

Original context in the discussion issue: #3805 (closed)

Draft ready in gitlab-org/gitlab!166729 (merged). Blocked on #3834 (closed).

Implementation considerations

1. Self-managed users using single-database or non-decoposed database setups should not be affected. (#3805 (comment 2114180394))
2. Sidekiq workers with urgency :high should not be overly if a database is set to sticky/delayed. (See gitlab-org/gitlab!167877 (merged))
3. Rollout using feature flag should be supported.