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/appContext.ts

@@ -0,0 +1,28 @@
+/*---------------------------------------------------------------------------------------------
+ *  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 vscode from 'vscode';
+import { ApiWrapper } from './apiWrapper';
+
+/**
+ * Global context for the application
+ */
+export class AppContext {
+
+    private serviceMap: Map<string, any> = new Map();
+    constructor(public readonly extensionContext: vscode.ExtensionContext, public readonly apiWrapper: ApiWrapper) {
+        this.apiWrapper = apiWrapper || new ApiWrapper();
+    }
+
+    public getService<T>(serviceName: string): T {
+        return this.serviceMap.get(serviceName) as T;
+    }
+
+    public registerService<T>(serviceName: string, service: T): void {
+        this.serviceMap.set(serviceName, service);
+    }
+}