/* Cancel outstanding rest requests.
:param category: if specified, only abort those requests that have
xhr.girder[category] set to a truthy value.
function cancelRestRequests(category) {
    _.each(restXhrPool, function (xhr) {
        if (category && (!xhr.girder || !xhr.girder[category])) {
            return;
        }
        if (xhr.abort) {
            xhr.abort();
        }
    });
}