Implement Cache on getTreeList in DBTreeListClass

The following function can be called a lot in livetables for TreeList fields. The Map that we pass to it is retrieved from cache but the result of getTreeList is not and it can be time consuming to generate the data from getTreeList which has no reason to change.

protected List<ListItem> getTreeList(Map<String, List<ListItem>> treemap, Map<String, ListItem> map,
XWikiContext context)

{ List<ListItem> list = new ArrayList<ListItem>(); addToTreeList(list, treemap, map, "", context); return list; }