db_bench: add a mode to operate multiple DBs

Summary: This patch introduces a new parameter num_multi_db in db_bench. When this parameter is larger than 1, multiple DBs will be created. In all benchmarks, any operation applies to a random DB among them. This is to benchmark the performance of similar applications.

Test Plan: run db_bench on both of num_multi_db=0 and more.

Reviewers: haobo, ljin, igor

Reviewed By: igor

CC: igor, yhchiang, dhruba, nkg-, leveldb

Differential Revision: https://reviews.facebook.net/D17769