Fix unchecked_map destructor.

[clemahieu]

Fix an issue with unchecked_map::~unchecked_map where the condition variable signal can be missed since the object mutex isn't acquired while setting stopped=true.

[theohax]

In practice I doubt this can actually bite us, but yeah, the proper way of announcing changes via CVs is under the mutex indeed, even if the change is atomic like this one. Just curious, have you seen this cause issues?

[dsiganos]

@theohax In practice, it would bite us on node shutdown, the node could deadlock on shutdown.

[clemahieu]

If the working thread is between these two lines when stopped is set to true and the condition variable in notified, it will neither detect the stopped flag nor detect the condition variable notification.

	while (!stopped)
	{
		if (!buffer.empty ())