MB-100: graph rebalance progress in perf. reports

Change-Id: I840edc9c58df1c8480079d1a4db2e17066f9f808
Reviewed-on: http://review.couchbase.org/24462
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>