When formatting applyOps oplog entries for a multi-document transaction, the OpObserver implementation has to take into account how to update the config.images collection with pre/post images related to findAndModify operattions. In the packTransactionStatementsForApplyOps() function, we have a local function setImageToWrite that could be extracted into a method of the ReplOperation class.

Another improvement we could make is to promote the internal struct ImageBundle to a member struct of ReplOperation.