FileSystemConnector exposes child nodes that might not be accessible

The new FileSystemConnector is always adding child node references for all java.io.File objects returned from parentDir.listFiles(...). This is not correct; we should check whether the resulting File objects actually exist and are readable (e.g., no permission to read).

Note that in the case of a broken symlink (see MODE-1482), a File object will be returned from the listFiles(...) method but will not exist.