…calculation failure.

This is to protect the cluster assignment on a rebalancing algorithm failure. For example, the cluster is out of capacity. In this case, the rebalancer will keep using the previously calculated mapping.
Also, refine the new metric interface, and add the RebalanceFailureCount metric for recording the failures.

Modify the test cases so that DBs from different test cases have different name. This is to avoid previous test records to be returned by the rebalancer on calculation error.