/* Get the number of outstanding rest requests.
:param category: if specified, only count those requests that have
xhr.girder[category] set to a truthy value.
:returns: the number of outstanding requests.
function numberOutstandingRestRequests(category) {
    if (category) {
        return _.filter(restXhrPool, function (xhr) {
            return xhr.girder && xhr.girder[category];
        }).length;
    }
    return _.size(restXhrPool);
}