[filebrowser] change download behaviour and cache policy to the latest.

1. download will save the data to a tmp file and then rename it to the target
   file

2. change cache policy and it will check existence of the responsing local file
first instead of checking the match of oids first.

3. add a new method to the cache manager to checkout oid without modifying the
   existing data. use this very method since the oid which is returned by
   file-browser might be outdated.