Rollback occasionally fails to remove new attachments and replace with old.

This was discovered by a flicker in StorageTests: AttachmentTest#testRollbackAfterSaveDeleteSaveAttachment() the problem only seems to appear when an attachment is attached, then deleted, then another attachment is attached with the same name, then the user tries to rollback the version to the one with the original attachment. Sometimes the attachment does not revert.

This problem appears independent of whether or not the FilesystemAttachment store is used so (unless both attachment storage engines have the same problem) the attachment storage system can be absolved of guilt.