MultiCfIterator Impl Follow up (#12465)

MultiCfIterator Impl Follow up (#12465)

Summary:
As a follow up for https://github.com/facebook/rocksdb/issues/12422 , this PR includes the following two changes.
- Removal of `direction_` in the MultiCfIterator
- Use of Member Func Template instead of `std::function`

Pull Request resolved: https://github.com/facebook/rocksdb/pull/12465

Test Plan:
```
./multi_cf_iterator_test
```

Reviewed By: pdillinger, ltamasi

Differential Revision: D55208448

Pulled By: jaykorean

fbshipit-source-id: 8b3167c1d59839d076afc29097b5ad21a453460a