CBQE-0 add log to dcp rebalance

Change-Id: Ide7cd3f8252976433cb2e302c45af579c307cb05
Reviewed-on: http://review.couchbase.org/41409
Reviewed-by: Thuan Nguyen <soccon@gmail.com>
Tested-by: Thuan Nguyen <soccon@gmail.com>