Some small cleaning up to make some compiling environment happy

Summary: Compiler complains some errors when building using our internal build settings. Fix them.

Test Plan: rebuild

Reviewers: haobo, dhruba, igor, yhchiang, ljin

Reviewed By: igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17199
Fix valgrind issues

Summary:
NewFixedPrefixTransform is leaked in default options. Broken by https://github.com/facebook/rocksdb/commit/b47812fba601e23872349407d565d15f0b41a2fe

Also included in the diff some code cleanup

Test Plan:
valgrind env_test
also make check

Reviewers: haobo, danguo, yhchiang

Reviewed By: danguo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17211
Make rate limiting unit test more robust
allow mmap writes