[v6r7] DEV: RSSCache without thread in background ( Needs certification ! )

**Please, do not merge with other patches.**

Before, RSSCache was querying the server every X minutes. So far so god, for the agents... but the jobs are refreshing the cache even if they do not need it anymore.

Too many wasted queries.

Solution: query only if the cache is invalid or empty. 
Consequence: if the cache is cold, the first query to the cache will take little bit longer. But, as the servers are going to be much less loaded, the difference should be minimal.