Summary:
PosixLogger and AutoRollLogger do not seem to be thread safe.
For PosixLogger, log_size_ is not atomically updated.
For AutoRollLogger, the underlying logger_ might be deleted by
one thread while still being accessed by another.

Test Plan: make check

Reviewers: kailiu, dhruba, heyongqiang

Reviewed By: kailiu

CC: leveldb, zshao, sheki

Differential Revision: https://reviews.facebook.net/D9699