Delete Todos for guest users when project visibility
level is updated to private.

In the context of a Geo setup, some jobs can be
running on a Geo secondary where the database
is read-only and therefore we should guard
against various jobs attempting to write.

Introduce sidekiq worker for auto merge process

See merge request gitlab-org/gitlab-ce!28780

Make pipeline schedule worker resilient

Closes gitlab-com/gl-infra/production#805 and #61955

See merge request gitlab-org/gitlab-ce!28407

As we have a central domain for auto merge process today,
we should use a single worker for any auto merge process.

We have one auto merge strategy today - Merge When Pipeline
Succeeds.

In order to add more strategies for Merge Train feature,
we abstract the architecture to be more extensible.

Removed arguments

Fix spec

Currently, pipeline schedule worker is unstable because it's
sometimes killed by excessive memory consumption.
In order to improve the performance, we add the following fixes:

1. next_run_at is always real_next_run, which means the value
   always takes into account of worker's cron schedule
1. Remove exlusive lock. This is already covered by real_next_run
   change.
1. Use RunPipelineScheduleWorker for avoiding memory killer.
   Memory consumption is spread to the multiple sidekiq worker.

We've already migrated all the legacy artifacts to the new realm,
which is ci_job_artifacts table.
It's time to remove the old code base that is no longer used.