Basic Load Balancer Functionality

[livingshade]

Why are these changes needed?

Now we support client connect to multiple server ip address and a basic round-robin load balancer that distribute the request accordingly.

Changes

• Load balancer engine, which stores the connection info and rewrite the conn_id accordingly
• Mrpclb module, which is mrpc module with load balancer engine
• An example called load_balancer, you need to start two server first and then start client. Client will send request to each server.

Todos

• Reuse code of mrpc module in mrpclb module
• Error info
• Refactor connection structure

Checks

• [x] I've run scripts/clippy.sh to lint the changes in this PR.
• [ ] I've included any doc changes.
• Testing Strategy
  • [ ] Release tests
  • [x] This PR is not tested :(

[livingshade]

cc @crazyboycjr

[livingshade]

Can you merge this first? I feel like the refactor and message bus requires a lot of code change, and perhaps we should separate it from load balancer.