No code logic changes was done

a     -> gain
b     -> cost_of_building_range_filter
a_adj -> gain_adj
r     -> row_combinations

Other things:
- Optimized the layout of class Range_rowid_filter_cost_info.
  One effect was that I moved key_no to the private section to get
  better alignment and had to introduce a get_key_no() function.
- Indentation changes in rowid_filter.cc to avoid long rows.