Summary:
Current PosixLogger performs IO operations using posix calls. Thus the
current implementation will not work for non-posix env. Created a new
logger class EnvLogger that uses env specific WritableFileWriter for IO operations.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5491

Test Plan: make check

Differential Revision: D15909002

Pulled By: ggaurav28

fbshipit-source-id: 13a8105176e8e42db0c59798d48cb6a0dbccc965