check prefix_size when using hash search in db_bench

Summary:
1. Check prefix_size when enable use_hash_search in db_bench
2. Remove include/statistics.h in db_bench

Test Plan: ./db_bench --use_hash_search=1

Reviewers: ljin, yhchiang, igor, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D21375