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/PostgresStorageAdapter.spec.js

@@ -0,0 +1,22 @@
+const PostgresStorageAdapter = require('../src/Adapters/Storage/Postgres/PostgresStorageAdapter');
+const databaseURI = 'postgres://localhost:5432/parse_server_postgres_adapter_test_database';
+
+describe_only_db('postgres')('PostgresStorageAdapter', () => {
+  beforeEach(done => {
+    const adapter = new PostgresStorageAdapter({ uri: databaseURI })
+      .deleteAllClasses()
+      .then(() => {
+        adapter.handleShutdown();
+      }, fail)
+      .catch(done);
+  });
+
+  it('handleShutdown, close connection', (done) => {
+    const adapter = new PostgresStorageAdapter({ uri: databaseURI });
+
+    expect(adapter._client.$pool.ending).toEqual(false);
+    adapter.handleShutdown();
+    expect(adapter._client.$pool.ending).toEqual(true);
+    done();
+  });
+});