4.6 will introduce a new style of "delta" oplog entries used for updates. Since we may have to disable this functionality by default (in order to adhere to API versioning), we should make sure we have test coverage both for configurations with the default on and the default off.

How we achieve this is to be decided. One option would be to add passthroughs for core/ and replsets/ which set the flag. We may also want to change some of the generational fuzzers.