enable kHashSearch for blocktable in db_bench

Summary:
  add a flag called use_hash_search in db_bench

Test Plan:
  make all check
  ./db_bench --use_hash_search=1

Reviewers: ljin, haobo, yhchiang, sdong

Reviewed By: sdong

Subscribers: igor, dhruba

Differential Revision: https://reviews.facebook.net/D20067