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