Summary:
ScopedArenaIterator is not an iterator. It is a pointer wrapper. And we don't need a custom implemented pointer wrapper when std::unique_ptr can be instantiated with what we want.
So this adds ScopedArenaPtr<T> to replace those uses.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/12470
Test Plan: CI (including ASAN/UBSAN)
Reviewed By: jowlyzhang
Differential Revision: D55254362
Pulled By: pdillinger
fbshipit-source-id: cc96a0b9840df99aa807f417725e120802c0ae18