Implemented #18865: Faster cache clearing with eZDFS.

Added support for storing cache files in a separate 'ezdfsfilecache'
table. This table can then be truncated to allow faster cache clearing
instead of using like queries to empty the ezdfsfile table.

Discussion: http://share.ez.no/forums/general/faster-cache-clearing/
Issue: http://issues.ez.no/18865

# TODO
- [ ] Adapt the feature to the cluster index refactoring introduced in 2013.5