Do not clean cache for inivisble products

Hi, I changed the logic for cache-cleaning to skip invisible products. 

I noticed that my import got very slow. We have products with hundreds of variations (Simple Products) which are not visible to the site visitor, and thus are not view- and cache-able. But for every save action Lesti::FPC tells the Cache Backend  to run a cleaning process, which is quite heavy. I think this is unnecessary for invisible products, so added a simple check.