rollbackViaRefetch should not update documents in capped collections

In rollbackViaRefetch, when we roll back changes to a document and the document exists on the source, we perform an update on the document. This is problematic for capped collections, since updates in capped collections must not change the size of the document. For capped collections, we should instead truncate the collection (like we do for deletes) and then insert the source's version of the document.