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
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