Before this change we set the modtime of the cache file when all
writers had finished.
This has the unfortunate effect that the file is uploaded with the
wrong modtime which means on backends which can't set modtimes except
when uploading files it is wrong.
This change sets the modtime of the cache file immediately in the
cache and in turn sets the modtime in the file info.
Before this change the background writing of the file was racing with
the test of the object on the remote.
This meant that the tests passed locally but failed on a lot of the
remotes.
