create_missing_partitions() lock database during stories vacuum

[hroberts]

All of our crawler_fetcher and fetch_link workers were clocking on create_missing_partitions(). create_missing_partitions was blocked on an autovacuum of the stories table.

I'm not sure how to fix this long term. For the short term, I just disabled create_missing_partitions worker until the stories autovacuum finishes (the stories vacuum has already been running for a day, so I don't want to make it restart).