replace unique_ptr constructor with MakeUnique or UniqueIterator

[gogim1]

ref #711 (comment)
ref #703 (comment)

I noticed there are many places in the code base where used the unique_ptr constructor directly instead of using MakeUnique and UniqueIterator. Do we need to replace them all? I wonder if this fix is worth PRs as it is too trivial and doesn't seem to bring performance benefits.

[PragmaTwice]

I think it is OK to replace them to keep a unified code style.