Adding unique index using global sort is slow

[CbcWestwolf]

In an internal 200T adding index testing, we found that adding a normal index costs 2h30m, while adding a unique index costs 3h24m.
The extra time cost is in checkDuplicateForUniqueIndex, which is unnecessary for global sort.
Besides, we can detect duplicate on step read index, merge sort and cloud import, which helps to report the error in advance.