Dividing logger.
cleaning up loggerset.
removing fixme from State.
moving comment.
implementing renewLoggerSet.
adding doc.
for backup.
implementing rotator.
adopting lock-less algorithm.

Is this safe enough?
Revert "adopting lock-less algorithm."

This reverts commit 94a9fb12293fccaa51c514e306bbc8c0720ae8d0.
This patch does not contribute for logging speed.
Let's go back to the safer one.
removing comment.
adding comment.