db_bench compression options
db_bench compression options

Summary:
- moved existing compression options to `InitializeOptionsGeneral` since they cannot be set through options file
- added flag for `zstd_max_train_bytes` which was recently introduced by #3057
Closes https://github.com/facebook/rocksdb/pull/3128

Differential Revision: D6240460

Pulled By: ajkr

fbshipit-source-id: 27dbebd86a55de237ba6a45cc79cff9214e82ebc

bbiao/rocksdb,bbiao/rocksdb,Andymic/rocksdb,bbiao/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,Andymic/rocksdb,Andymic/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,Andymic/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,bbiao/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,Andymic/rocksdb,Andymic/rocksdb