[Tuning Contest] YCSB LevelDB Option Update

[min-guk]

I updated YCSB-cpp to add previously unsupported leveldb options. Now, You can modify all options of leveldb and run benchmarks without additional build.

Please, follow the description written in below link.

• https://github.com/DKU-StarLab/leveldb-study/blob/main/tuning/README.md

Thanks.