Patch handleShutdown feature (#4361)

* Add handleShutdown Test of MongoStorageAdapter.

* Add `handleShutdown` function in PostgresStorageAdapter, with it's test.

* Replace the _pgp to _client

* Use  `adapter._client.$pool.ending` to ckeck is ending

* Add `handleShutdown()` in ParseServer.spec.js

* Trigger

* Set Server to Default

spec/MongoStorageAdapter.spec.js

@@ -237,4 +237,23 @@ describe_only_db('mongo')('MongoStorageAdapter', () => {
         done();
       });
   });
+
+  it('handleShutdown, close connection', (done) => {
+    const adapter = new MongoStorageAdapter({ uri: databaseURI });
+
+    const schema = {
+      fields: {
+        array: { type: 'Array' },
+        object: { type: 'Object' },
+        date: { type: 'Date' },
+      }
+    };
+
+    adapter.createObject('MyClass', schema, {}).then(() => {
+      expect(adapter.database.serverConfig.isConnected()).toEqual(true);
+      adapter.handleShutdown()
+      expect(adapter.database.serverConfig.isConnected()).toEqual(false);
+      done();
+    });
+  });
 });