Improve performance of Taxon promotion rule

[gmacdougall]

1. Before #actionable? was bringing all product ids of the rule's taxons
   from the database. On a particular case in a store with 1mil
   products, the promotion was bringing 0.6mil of them, with a query ~ 5sec
   and 5mil ids in Ruby.

   After, the exists? query is a milliseconds one.

2. Before #eligible? was fetching all parent taxons of each taxon on a
   product, which is not required for the match process.

   So, given an order with 10 line items, with each line item having a
   product with 3 taxons, each of them having 2 parent taxons
   would mean 30 queries to the db to fetch the ancestors, bringing a
   total of 60 taxons in Ruby memory.

   All of them unnecessary, as the matching processes can be done solely
   based on the rule's taxons, which if 5 would mean 5 queries to the DB
   (self_and_descendants)

3. Fix incorrect test on #eligible?#all for taxon child of a taxon rule
   case.

   • there were no children tested in the case

4. On #eligible?#all, take only the IDs from the DB to
   reduce memory overhead

This is a rebased version of #1409