Shapefile renderer opens and scans thru the DBF file even if no attribute needs to be read

The shapefile renderer opens the dbf file to grab the header, and then again moving from feature to feature during rendering, even if the attribute analisys shows no attribute needs to be read. This slows down geoserver map production by 10% on single threaded tests, and as much as 30% when 60 threads are accessing geoserver simultaneously.
Avoiding to open the DBF file was one of the first streaming renderer optimization, and it's still there, shapefile renderer should have it too.

I'm committing a patch to avoid dbf file opening, as usual I'd like a review.