[FileBackend] Added "adviseStat" option for the "listing followed by stat" case.

* Used this parameter to speed up copyFileBackend.php.
* Also added mtime checks to copyFileBackend.php and a few cleanups.
* Also fixed some incorrect getDirListPageInternal/getFileListPageInternal docs.

Change-Id: I424ef238f7adf4cf1f33b74e3a4e187dcb328a99
(cherry picked from commit d1940caa1f4a11e7c7d8d2ff5b178da7de8e2a92)