The rdpContext gets an event which will
get set if an error occoured in a channel.
If a thread or a void callback has to report an
error it will get signaled by this system.
1. Introduce message queue in shadow client. No longer use subsytem->MsgPipe->out to deliver message to clients.
We used to use subsytem->MsgPipe->out for messages which need to be sent to client. But it's not correct. Only one client would get the message if multiple client exists
This problem make the fix in PR #2643 incomplete.
Introduced reference count based solution to release resource taken by the message.
Also added APIs for client message delivery.
Also fixed msg pipe in subsystem to clean resource when destroyed.
2. Discard unused StopEvent in client. We actually use quit message instead.
3. Enhance disposal of channels.
Free context for remdesk and encomsp channels. The original fix only stop the threads, but doesn't release resource.
Dispose channels earlier. The channels are built on client->vcm. Disposing channels after client->vcm is closed cause unknown behavior.
Original fix is #2644
4. Start to add audio support.
