Make db_bench less space for --stats_per_interval
Make db_bench less space for --stats_per_interval

Summary:
Changes compaction IO stats to be printed once per interval rather
than once per interval per thread. https://github.com/facebook/rocksdb/issues/1276

Task ID: #12698508

Test Plan: run db_bench

Reviewers: sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D62067

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