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

src/Adapters/Storage/Postgres/PostgresStorageAdapter.js

@@ -574,6 +574,13 @@ export class PostgresStorageAdapter {
     this._pgp = pgp;
   }
 
+  handleShutdown() {
+    if (!this._client) {
+      return
+    }
+    this._client.$pool.end();
+  }
+
   _ensureSchemaCollectionExists(conn) {
     conn = conn || this._client;
     return conn.none('CREATE TABLE IF NOT EXISTS "_SCHEMA" ( "className" varChar(120), "schema" jsonb, "isParseClass" bool, PRIMARY KEY ("className") )')