[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