Added Unified connection support (#3785)

* Added Unified connection support

* Use generic way to do expandNode.
Cleanup the ported code and removed unreference code. Added as needed later.
Resolved PR comments.

* Minor fixes and removed timer for all expanders for now. If any providers can't response, the tree node will spin and wait. We may improve later.

* Change handSessionClose to not thenable.
Added a node to OE to show error message instead of reject. So we could show partial expanded result if get any.
Resolve PR comments

* Minor fixes of PR comments

extensions/mssql/src/localizedConstants.ts

@@ -0,0 +1,13 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the Source EULA. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+'use strict';
+
+import * as nls from 'vscode-nls';
+const localize = nls.loadMessageBundle();
+
+// HDFS Constants //////////////////////////////////////////////////////////
+export const msgMissingNodeContext = localize('msgMissingNodeContext', 'Node Command called without any node passed');
+export const msgTimeout = localize('connectionTimeout', 'connection timed out. Host name or port may be incorrect');