Fsync directory after we create a new file

Summary:
@dhruba, I'm not sure where we need to sync the directory. I implemented the function in Env() and added the dir sync just after we close the newly created file in the builder.

Should I also add FsyncDir() to new files that get created by a compaction?

Test Plan: Confirmed that FsyncDir is returning Status::OK()

Reviewers: dhruba, haobo

Reviewed By: dhruba

CC: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D14751