[1.0.0] Optimize server side sort on sqlite / mysql

[ChadSikorra]

Two things related to sorting here:

1. Add an index to speed up server side sorting on sqlite. Without this index, sorting on a larger directory can be verrrry slow on sqlite.
2. Fix MySQL server-side sort to evaluate the sort key once per row instead of twice. This meant actual dialect / query changes.

Server side sort in general isn't super performant. But this at least makes it more usable than it was. Also added search-sort to the CI load test default mix so it's easier to spot these issues.