static DocumentOperationMessageV3 newRemoveMessage(FeedOperation op, String operationId) {
        var msg = new RemoveDocumentMessage(op.getDocumentRemove());

        String id = (operationId == null) ? msg.getDocumentId().toString() : operationId;
        return new DocumentOperationMessageV3(id, msg);
    }
Keeps an operation with its message.

@author Haakon Dybdahl
vespaclient-container-plugin/src/main/java/com/yahoo/vespa/http/server/DocumentOperationMessageV3.java