Draft: Inventory API: Do not use regex for measure

concat(stock.variation_text,'\n') REGEXP measure.variation is slow. This introduces an alternative approach of indexing the first 4 variations directly in columns and use a traditional join instead or a regexp match. In a real production case of a 2 seconds query, this was one order of magnitude faster.

This has some limitations:

• it does not support more than 4 variations
• it does not support optional variations

The previous regexp based implementation is kept in this patch, so that it's easy to rollback this change if side effects are found.