@Override
    public void deleteAllNodes() {
        explorerTreeDao.removeAll();
        addNode(null, ExplorerConstants.SYSTEM_DOC_REF, null);
    }
General form a function that returns a list of explorer nodes from the tree DAO

@param folderDocRef  The root doc ref of the query
@param fetchFunction The function to call to get the list of ExplorerTreeNodes given a root ExplorerTreeNode
@return The list of converted ExplorerNodes
stroom-explorer/stroom-explorer-impl/src/main/java/stroom/explorer/impl/ExplorerNodeServiceImpl.java