Reduce server log verbosity at info level

Currently, the broker logs two messages at INFO level that should be at a lower level. This serves only to fill up log files on disk, and can cause performance issues due to synchronous logging as well.

The first is the "Closing socket connection" message when there is no error. This should be reduced to debug level. The second is the message that ZkUtil writes when updating the partition reassignment JSON. This message contains the entire JSON blob and should never be written at info level. In addition, there is already a message in the controller log stating that the ZK node has been updated.