There are some bugs in Nereids' StatsCalculator.
1. Project: return child column stats directly, so its parents cannot find column stats from project's slot.
2. Aggregate: do not return column that is Alias, its parents cannot find some column stats from Aggregate's slot.
3. All: use SlotReference as key of column to stats map. So we need change SlotReference's equals and hashCode method to just using ExprId as we discussed.
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