Updates to index-rebalance and SYNC message.

Change-Id: Ib0710e96464c97a210e5b6445f7b76ec515e2835
Reviewed-on: http://review.couchbase.org/35190
Reviewed-by: John Liang <john.liang@couchbase.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>