Change configurable-children linking to avoid mysql limits

[palviggi-flagbit]

It changes the linking of configurables to use a sub query for each configurable, to avoid the limit of 1024 characters that group_concat has.

It was tested on a store that has a few dozens of configurables with ~3000 children each, and it finish in a couple of minutes.

It should probably be tested on a configuration with many configurables with few children each, but it's just an extra query per model.

Fixes #242