Fix file issue 

@ongardie 
For our raft implementation, we do fsync every time the follower append log entries from followers.
But we do not do fsync every time the leader receive a command from the clients.
I observer that leader does not need to do a fsync before actual committing. This is because the leader will decide when to commit the log entry. Thus the leader can safely assume that itself has the log entry no matter the log has been fsynced or not. 
Fsync every time the leader receiving a command will affect performance largely. 
Do you think it is a right observation?

/cc @baruch @benbjohnson