[WIP] [minor] add rebalancer implementation

[kpango]

Signed-off-by: kpango i.can.feel.gravity@gmail.com

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]
• 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:

• I have read the CONTRIBUTING document.
• 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.