refactor: use Vec<Vec<String>> as query results & fix hash-threshold

[xxchan]

• Breaking change to DB trait
• fix hash-threshold's behavior differs from original sqllogictest #107
• fix statement count n doesn't work for postgres engine #103
• verify column in option is the same as provided input #36 is now possible, but I didn't enable it yet. I'd like to implement generate test result using actual output of a run #43 first so that we can update painlessly.
• Breaking change: only sort actual results now. We need to sort before compute hash. And sorting Vec<Vec<String>> is different from sorting rows Vec<String>. Note that because of this, risingwave cannot use this version yet.. So we quite need generate test result using actual output of a run #43

[wangrunji0408]

LGTM 🚀