Implement Session support through the extension host (#3228)

Full plumb through of Session support. Also fixed some test issues

- Load session and get necessary information in kernels list
- Run Cell button now works as expected
- Added a ToggleAction base class which can be used for anything that switches icons. I'd still prefer to have this be dynamic and as clean as the extension classes
- Fixed account test unhandled promise rejections (caused by incorrect / invalid tests) that made it hard to see all the test run output.

src/sql/parts/notebook/models/clientSession.ts

@@ -299,7 +299,7 @@ export class ClientSession implements IClientSession {
 	public async shutdown(): Promise<void> {
 		// Always try to shut down session
 		if (this._session && this._session.id) {
-			this.notebookManager.sessionManager.shutdown(this._session.id);
+			await this.notebookManager.sessionManager.shutdown(this._session.id);
 		}
 		let serverManager = this.notebookManager.serverManager;
 		if (serverManager) {