{DCP}: Rebalance Exit with dcp_wait_for_data_move_failed after we stop swap rebalance twice (using different priority of buckets)

1166, Centos 6x, Ubuntu 12.04

1. Create 5 Node cluster (10.6.2.144-10.6.2.148)
2. Create 3 standard buckets, 2 sasl buckets
2 standard, 1 sasl have high priority and 1 standard, 1 sasl has lower priority
3. Add 100 K items to each bucket
4. Swap rebalance out 2 nodes (out:: 10.6.2.145, 10.6.2.146 in:: 10.6.2.149, 10.6.2.150)
5. Stop Rebalance at 20 % and continue rebalance
6. Stop Rebalance at 40 %

After Step 6, it was observed that attempt to rebalance would exit with the following message

Rebalance exited with reason {unexpected_exit,
{'EXIT',<0.22236.1>,
{dcp_wait_for_data_move_failed,
"standard_bucket1",487,'ns_1@10.6.2.144',
['ns_1@10.6.2.148','ns_1@10.6.2.146'],
{error,no_stats_for_this_vbucket}}}}

Note that this issue is not observed when buckets have same priority