[WIP] [minor] add rebalancer implementation

[kpango]

Signed-off-by: kpango [email protected]

Description:

implementation of rebalancer @vankichi @kevindiu @hlts2 please develop rebalancer using this base PR.

Related Issue:

How Has This Been Tested?:

Environment:

• Go Version: 1.15.2
• Docker Version: 19.03.8
• Kubernetes Version: 1.18.2
• NGT Version: 1.12.1

Types of changes:

• [ ] Bug fix [type/bug]
• [x] New feature [type/feature]
• [ ] Add tests [type/test]
• [ ] Security related changes [type/security]
• [ ] Add documents [type/documentation]
• [ ] Refactoring [type/refactoring]
• [ ] Update dependencies [type/dependency]
• [ ] Update benchmarks and performances [type/bench]
• [ ] Update CI [type/ci]

Changes to Core Features:

• [ ] Have you added an explanation of what your changes do and why you'd like us to include them?
• [ ] Have you written new tests for your core changes, as applicable?
• [ ] Have you successfully ran tests with your changes locally?

Checklist:

• [x] I have read the CONTRIBUTING document.
• [x] I have checked open Pull Requests for the similar feature or fixes?
• [ ] I have added tests and benchmarks to cover my changes.
• [ ] I have ensured all new and existing tests passed.
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have updated the documentation accordingly.