Summary:
Switch from std::unordered_map to RocksDB UnorderedMap for all the places that logging user-defined timestamp size in WAL used.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11507

Test Plan:
```
make all check
```

Reviewed By: ltamasi

Differential Revision: D46448975

Pulled By: jowlyzhang

fbshipit-source-id: bdb4d56a723b697a33daaf0f856a61d49a367a99