Expose a method to remove a stale reference for subclasses that overr…

[anuraaga]

…ide expungeStaleEntries.

As far as I can tell, there isn't a proper way of a subclass extending and overriding expungeStaleEntries since there's no way to actually remove the reference from the map, so it's a guaranteed memory leak. This exposes the removal for subclasses.

/cc @adriancole