fix bug #1977: remove etags from database after version rollback so that the client can detect the changes

update the filecache by setting mtimes to 0 when the mount configuration has changed instead of clearing the cache

etag has to be removed after version rollback to enable the sync client to detect the changes (bug #1829)

This should also fix the dirname problem on windows

Conflicts:
	lib/filesystemview.php

getETagPropertyForFile -> getETagPropertyForPath
removeETagPropertyForFile -> removeETagPropertyForPath

This is based on the md5 of the file, can be changed later

Added new readdir() method to all storage classes and handlers (only working implementation in local.php)