Add fetchRow DB function.

Helper function to fetch a row at a time using a cursor.

lib/www/server/lib/db/connection.js

@@ -49,9 +49,31 @@ async function schema2pid (schema, client) {
 	return res.rows[0] && res.rows[0].pid;;
 }
 
+/** Fetch one row from a database cursor.
+ *
+ * @a cursor A query cursor
+ *
+ * @returns A row object if there are any rows left to consume.
+ * @returns Undefined when all the rows (if any) have been consumed.
+ */
+async function fetchRow (cursor) {
+	return new Promise((resolve, reject) => {
+		cursor.read(1, (err, rows) => {
+			if (err) {
+				reject(err);
+			} else if (rows.length) {
+				resolve(rows[0]);
+			} else {
+				resolve(); // undefined
+			}
+		});
+	});
+}
+
 module.exports = {
 	pool,
 	transaction,
 	setSurvey,
-	schema2pid
+	schema2pid,
+	fetchRow
 };