db_bench cleanup

Summary:
clean up the db_bench a little bit. also avoid allocating memory for key
in the loop

Test Plan:
I verified a run with filluniquerandom & readrandom. Iterator seek will be used lot
to measure performance. Will fix whatever comes up

Reviewers: haobo, igor, yhchiang

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17559