rollback_set_fcv.js repeatedly tries to rollback changes to the fcv document, so we periodically want to check the target FCV values. While we're in rollback, we should access the FCV document directly instead of relying on the checkFCV function, which reads from the majority committed snapshot. Since this test cannot always guarantee that the fcv document changes make it into the majority committed snapshot, we should audit usages of this helper and when appropriate, change the code to access the document directly on the rollback node.