Graceful disconnect to remote Management Host

[MarkZuber]

During a remote connection, if the client closes gracefully, the closing message doesn't necessarily reach the Management host since the message is sent async. So the Management host sticks around expecting the possibility of a reconnection that isn't going to happen, because if the client window is fully closed then they'll do a full connection flow the next time they connect instead of a reconnect. If the disconnect message is not received by the Management host in vscode-server, then it hangs around for a long time and consumes remote resources.

During the window close / disconnect we want to ensure the message is sent immediately to the Management host before shutdown so that the Management host can fully close properly.