Optimize touch_taxons on product.
Optimize touch_taxons on product.

This will touch all of the taxons and taxon ancestors for  a product
using a constant number of updates (2, one for taxons, one for
taxonomies).

Previously the number of updates would be 2 * the number of taxons &
taxon ancestors, as it would update the taxonmy for each taxon on each
update.

Fixes #4636

Hates/spree,bjornlinder/Spree,Hates/spree,bjornlinder/Spree,Hates/spree,Hates/spree,bjornlinder/Spree