app: add export route

Currently, the export route only creates the snapshot (the other side
of in-place import). In the future, the export route can export to a
custom backup config (like import).

src/apps.js

@@ -33,6 +33,7 @@ exports = module.exports = {
 
     restore,
     importApp,
+    exportApp,
     clone,
 
     update,
@@ -1549,6 +1550,26 @@ function importApp(app, data, auditSource, callback) {
     });
 }
 
+function exportApp(app, data, auditSource, callback) {
+    assert.strictEqual(typeof app, 'object');
+    assert.strictEqual(typeof callback, 'function');
+
+    const appId = app.id;
+
+    let error = checkAppState(app, exports.ISTATE_PENDING_BACKUP);
+    if (error) return callback(error);
+
+    const task = {
+        args: { snapshotOnly: true },
+        values: {}
+    };
+    addTask(appId, exports.ISTATE_PENDING_BACKUP, task, (error, result) => {
+        if (error) return callback(error);
+
+        callback(null, { taskId: result.taskId });
+    });
+}
+
 function purchaseApp(data, callback) {
     assert.strictEqual(typeof data, 'object');
     assert.strictEqual(typeof callback, 'function');