Fix Celery worker --max-tasks-per-child for Celery 4.x. (#3625)
Fix Celery worker --max-tasks-per-child for Celery 4.x. (#3625)

* Fix Celery worker CLI parameter name that was changed in Celery 4.x.

* Set Celery worker --max-memory-per-child to 1/4th of total system memory.

* Review fixes.

* Review fixes.

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