Add -enable_pipelined_write to db_bench and add two defaults
Add -enable_pipelined_write to db_bench and add two defaults

Summary:
Expose pipeline write in db_bench and change the default to parallel memtable inserts
Closes https://github.com/facebook/rocksdb/pull/2527

Differential Revision: D5359825

Pulled By: siying

fbshipit-source-id: e30755feb07ff19a731c4058acf101e02de4e197

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