db_bench: introduce --benchmark_read_rate_limit
db_bench: introduce --benchmark_read_rate_limit

Summary:
Add the parameter in db_bench to help users to measure latency histogram with constant read rate.
Closes https://github.com/facebook/rocksdb/pull/1683

Differential Revision: D4341387

Pulled By: siying

fbshipit-source-id: 1b4b276

SunguckLee/RocksDB,Andymic/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,vmx/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,bbiao/rocksdb,bbiao/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,bbiao/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,bbiao/rocksdb,bbiao/rocksdb,vmx/rocksdb,bbiao/rocksdb,Andymic/rocksdb,bbiao/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,Andymic/rocksdb,Andymic/rocksdb,vmx/rocksdb,wat-ze-hex/rocksdb,wat-ze-hex/rocksdb,wat-ze-hex/rocksdb,Andymic/rocksdb,vmx/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,Andymic/rocksdb,vmx/rocksdb,SunguckLee/RocksDB