Junkai Xue

### Issues - [X] My PR addresses the following Helix issues and references them in the PR description: resolve #2821 ### Description - [X] Here are some details about my...

Support Helix Gateway Service for: 1. define protobuf for the service 2. implement liveness detection, session handle over. 3. implement failover mechanism

Build scalability testing environment to benchmark: 1. Number of application connections. 2. Memory / CPU usage of max connections. 3. Frequent connect/disconnect. 4. Meta data synchronization.

Functional test for Gateway service including: 1. Standard state transitions. 2. Edge cases such as failover and connection loss 3. Convert testing to continuously certifications.

Support a new rebalanced to satisfy: 1. Assignment at creation. 2. Trigger assignment recompute based on configuration. 3. Allow pluggable movement strategy. 4. Default movement strategy implementation (threshold based)

Define the new set of public APIs for Admin. Define the simplified APIs for HelixManager.

Get documentation ready at least for: 1. design of gateway service. 2. design APIs. 3. user guide