Use the shared method in Ci::Pipeline

If pipeline schedule is to run at the exact same time with when cron
worker runs, the pipeline schedule will not be executed at the
ideal timing.

We fix this bug by comparing the exact matching of ideal and
cron worker's next run at.

This reverts merge request !27503

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.

- Renames attributes from stat to project_statistiscs_name and attribute
to statistic_attribute
- Reordes methods on UpdateProjectStatistics concern
- Removes unused module from Ci::Build

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.