Rebalance fails with "had_backfill,30000" timeout during rebalance-in 3 nodes on a 10 bucket cluster.

Repro issue using below test.
./testrunner -i /tmp/4-u-nodes.ini get-cbcollect-info=True -t rebalance.rebalancein.RebalanceInTests.rebalance_in_with_ops,nodes_in=3,items=0,default_bucket=false,standard_buckets=5,sasl_buckets=5

Steps
1. Create 10 buckets on a single node.
2. Add 3 nodes and rebalance in these nodes
3. At around 90% rebalance completion, the rebalance fails with a timeout error on had_backfill.

• Seeing this with every 3.0 run
• The exact same test runs clean on 2.5 previous builds.

Attaching logs.

https://s3.amazonaws.com/bugdb/MB-10036/bug_rebal.tar