Full auto rebalancer with CrushEd algorithm changed partition assignment unexpected.

[jiajunwang]

It was reported and verified that the current full auto rebalancer with CrushEd strategy may change partition assignment if the rebalance is triggered rapidly (2 or 3 times per second). This caused unexpected partition shuffling.

Till now, based on our investigation, the problem might be related to the unstable map key iterator in Java 8 or after. Since this usually happens with a short while, it might be overlooked since we bump up JDK version.

We plan to fix it while ensuring the algorithm's backward compatibility.