Implement the smooth weighted round-robin balancing for pingora.

## What is the problem your feature solves, or the need it fulfills?

The current weighted round-robin balancing algorithm does not ensure smoothness, which can result in requests being distributed to the same backend in short term.

## Describe the solution you'd like

By referring to the implementation of nginx, we can solve this problem.

## Describe alternatives you've considered

See https://github.com/cloudflare/pingora/pull/194 please.