Add an option to table_reader_bench to access the table from DB And Iterating non-existing prefix case.

Summary: This patch adds an option to table_reader_bench that queries run against DB level (which has one table). It is useful if user wants to see the extra costs DB level introduces.

Test Plan: Run the benchmark with and without the new parameter

Reviewers: haobo, dhruba, kailiu

Reviewed By: kailiu

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13863