Remove serialize

As ask in Issue https://github.com/GordonLesti/Lesti_Fpc/issues/60 here are my changes to remove serialize functions.

After installing this new changes make sure to flush the caches.

I'll also removed some duplicated code.

Thanks for merging!
@SchumacherFM Comes late into my mind, but this will only work for shops between 09-Sep-2001 and 11-Nov-2286 or? Fortunately Magento2 will be released before 2286.