Extensibility: Context menu support in Object Explorer (#1883)

- Fixes #1867 context menu should be extensible
- Added context keys to support "when" conditions on the new extensions
- Fixes issue where actions like New Query, scripting show up even if these are not valid for the provider type or object type
- Fixed node expansion bug where rapid connect / expand / disconnect could break the app (fix in ObjectExplorerService.onNodeExpanded)
- Major change to how internal actions work. These cannot assume the context has non-serializable objects. Opened up some APIs to make this easier to handle.
- Fixed a number of existing bugs in internal actions.
  - Notably, DisconnectAction was adding a listener on each right-click on an active connection and never getting it disposed. This wasn't needed at all due to design changes.
  - Another bug fix is that the Manage action now correctly navigates to the DB dashboard for database-level connections. Before this it went to the server-level dashboard.

* Define API for context info

src/sql/parts/objectExplorer/common/objectExplorerService.ts

@@ -162,10 +162,16 @@ export class ObjectExplorerService implements IObjectExplorerService {
 			error(expandResponse.errorMessage);
 		}
 
-		let nodeStatus = this._sessions[expandResponse.sessionId].nodes[expandResponse.nodePath];
-		if (nodeStatus && nodeStatus.expandEmitter) {
-			nodeStatus.expandEmitter.fire(expandResponse);
-		} else {
+		let sessionStatus = this._sessions[expandResponse.sessionId];
+		let foundSession = false;
+		if (sessionStatus) {
+			let nodeStatus = this._sessions[expandResponse.sessionId].nodes[expandResponse.nodePath];
+			foundSession = !!nodeStatus;
+			if (foundSession && nodeStatus.expandEmitter) {
+				nodeStatus.expandEmitter.fire(expandResponse);
+			}
+		}
+		if (!foundSession) {
 			warn(`Cannot find node status for session: ${expandResponse.sessionId} and node path: ${expandResponse.nodePath}`);
 		}
 	}