introduce TestMemEnv and use it in db_test

Summary:
TestMemEnv simulates all Env APIs using in-memory data structures.
We can use it to speed up db_test run, which is now reduced ~7mins when it is
enabled.
We can also add features to simulate power/disk failures in the next
step
TestMemEnv is derived from helper/mem_env
mem_env can not be used for rocksdb since some of its APIs do not give
the same results as env_posix. And its file read/write is not thread safe

Test Plan:
make all -j32
./db_test
./env_mem_test

Reviewers: sdong, yhchiang, rven, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28035