FIX: Flush was clearing all file-based caches

[This problem](https://github.com/silverstripe/silverstripe-framework/issues/3122) just won’t seem to go away :(

The issue is [here](https://github.com/silverstripe/silverstripe-framework/blob/3.1/thirdparty/Zend/Cache/Backend/File.php#L682). As the template partial cache data isn’t tagged, an empty array is passed for the tags when cleaning [here](https://github.com/silverstripe/silverstripe-framework/blob/3.1/view/SSViewer.php#L958). This means that every bit of data stored in a file-based cache will be matched, so data from all caches will be cleared.

I’ve captured the issue in a test now, so hopefully we’ve seen the last of this :)