Add readrandom with both memtable and sst regression test

Summary: @MarkCallaghan's tests indicate that performance with 8k rows in memtable is much worse than empty memtable. I wanted to add a regression tests that measures this effect, so we could optimize it. However, current config shows 634461 QPS on my devbox. Mark, any idea why this is so much faster than your measurements?

Test Plan: Ran the regression test.

Reviewers: MarkCallaghan, dhruba, haobo

Reviewed By: MarkCallaghan

CC: leveldb, MarkCallaghan

Differential Revision: https://reviews.facebook.net/D14511