Abstract renderer stores rendered item index indefinitely

Steps to Reproduce

http://zkfiddle.org/sample/4kan5f/2-zk-renderer-mem-leak

Current Result

The size of the "$RENDERED_COMPONENTS$" attribute grows forever with each render event.
The content of the map doesn't seem to be used and only contains pairs such as

,

etc.

Expected Result

Renderer shouldn't maintain an expensive object unless there is a specific need. shallow debug couldn't find a use case for the map that could be replaced by a "last index" attribute instead which would just increment a single value instead of storing every past index

Debug Info

AbstractRenderer have the main implementation
https://github.com/zkoss/zk/blob/master/zkbind/src/org/zkoss/bind/impl/AbstractRenderer.java#L91-L118
ChildrenBindingRenderer have a specialized implementation
https://github.com/zkoss/zk/blob/1e82bea329ea6d627aafb435ed192e1afb85c7e5/zkbind/src/org/zkoss/bind/impl/BindChildRenderer.java#L169-L196

Root Cause

Workaround

replace the hashmap by a LRUMap or any other Map implementation with a max size