Kickoff for the Weight Aware Globally Even Distribute Rebalancer

[jiajunwang]

We will soon start the development of the new rebalancer. https://github.com/apache/helix/wiki/Design-Proposal---Weight-Aware-Globally-Even-Distribute-Rebalancer

This issue is created for tracking the preparation.

• Create a branch and check in the initial code based on the POC.
• Create all the related tasks in the form of issues and assign the work.

[jiajunwang]

With some discussion with the team, we will create the issues later by the task owner.

Here's a rough plan for who will mainly contribute to this new rebalancer.
@jiajunwang - API/configure + Pipeline integration
@narendly - Change Detector + Assignment Metadata Store
@i3wangyi - Rebalance algorithm optimization + integration test + rebalancer performance evaluation

[jiajunwang]

The foundation of the new rebalancer has been built after the interface/cluster model checked in. I'm closing this issue since we will have separate issues to track the following components development.