Adding HTTP Headers "No-Cache" while using stale cache

If we use the Cluster eZDFSFileHandler, eZ enables thes StaleCache feature to give old cache of contents when a client ask a page whose new cache is generating...
But if we use eZ behind a Reverse Proxy like Varnish, the proxy will store the obsolete content within its own cache, unless we tell him to not do it.

This PR adds a HTTP Header when the page is rendered from stale cache so that Varnish will not store the wrong content into its own cache.