fix possible concurrent modification exception

Author: parg

core/src/com/biglybt/core/impl/CoreImpl.java

@@ -226,7 +226,7 @@
 
 	private boolean					ll_started;
 
-	private final List<CoreOperationListener>				operation_listeners		= new ArrayList<>();
+	private final CopyOnWriteList<CoreOperationListener>	operation_listeners		= new CopyOnWriteList<>();
 	private final CopyOnWriteList<CoreOperation>			operations				= new CopyOnWriteList<>();
 
 	private final CopyOnWriteList<PowerManagementListener>	power_listeners = new CopyOnWriteList<>();