Navigation services should support Last-Modified/If-Modified-Since Headers

AtomPub and Browser navigation services including getChildren, getFolderTree, getDescendants, and potentially the others should support Last-Modified/If-Modified-Since Headers.

Some repositories have built-in caching mechanisms to track whether a folder tree has been modified. Since many of the Navigation services often require significant server-side resources, repositories with these built-in caching mechanisms should be able to leverage them with OpenCMIS. Currently, the OpenCMIS NavigationService does not provide hooks to implement such optimizations.