db_bench to support rate limiter
db_bench to support rate limiter

Summary: Add --rate_limiter_bytes_per_sec to db_bench to allow rater limit to disk

Test Plan:
Run
./db_bench --benchmarks=fillseq --num=30000000 --rate_limiter_bytes_per_sec=3000000 --num_multi_db=8 -disable_wal
And see io_stats to have the rate limited.

Reviewers: yhchiang, rven, anthony, kradhakrishnan, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D38385

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