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/objectExplorerNodeProvider/cancelableStream.ts

@@ -0,0 +1,27 @@
+/*---------------------------------------------------------------------------------------------
+ *  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 { Transform } from 'stream';
+import * as vscode from 'vscode';
+import * as nls from 'vscode-nls';
+
+const localize = nls.loadMessageBundle();
+
+export class CancelableStream extends Transform {
+	constructor(private cancelationToken: vscode.CancellationTokenSource) {
+		super();
+	}
+
+	public _transform(chunk: any, encoding: string, callback: Function): void {
+		if (this.cancelationToken && this.cancelationToken.token.isCancellationRequested) {
+			callback(new Error(localize('streamCanceled', 'Stream operation canceled by the user')));
+		} else {
+			this.push(chunk);
+			callback();
+		}
+	}
+}