A simple benchmark to measure WAL append latency

Summary: A simple benchmark that simulates WAL append. It can be used to test different platform/file system's performance on WAL.

Test Plan: run it.

Reviewers: haobo, kailiu

Reviewed By: haobo

CC: igor, dhruba, i.am.jin.lei, yhchiang, leveldb, nkg-

Differential Revision: https://reviews.facebook.net/D16239