chore: refactor WBWIMemTable::NewIterator for better readability

[mdqst]

I refactored the WBWIMemTable::NewIterator function to improve its clarity and memory management. Instead of creating a unique_ptr inline in the constructor, I used std::make_unique to create the iterator and passed it more clearly using std::move.

This change makes the code simpler, more readable, and ensures proper ownership transfer of the iterator.