Implement the smooth weighted round-robin balancing for pingora.

[wathenjiang]

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 #194 please.