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.