[lldb] [llgs] Implement non-stop style stop notification packets

Implement the support for %Stop asynchronous notification packet format
in LLGS.  This does not implement full support for non-stop mode for
threaded programs -- process plugins continue stopping all threads
on every event.  However, it will be used to implement asynchronous
events in multiprocess debugging.

The non-stop protocol is enabled using QNonStop packet.  When it is
enabled, the server uses notification protocol instead of regular stop
replies.  Since all threads are always stopped, notifications are always
generated for all active threads and copied into stop notification
queue.

If the queue was empty, the initial asynchronous %Stop notification
is sent to the client immediately.  The client needs to (eventually)
acknowledge the notification by sending the vStopped packet, in which
case it is popped from the queue and the stop reason for the next thread
is reported.  This continues until notification queue is empty again,
in whic...