change on-disk log layout to {log.dir}/topicname/partitionid

Currently, the on-disk layout is

{log.dir}/topicname-partitionid. The problem is that there is no appropriate place to store topicname level information such as topic version. An alternative layout is {log.dir}

/topicname/partitionid. Then, we can store topic level meta data under

{log.dir}

/topicname.