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.
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