round-robin balancer has a thread-safety problem

[biningo]

Hi~ @zehuamama
I found that round-robin balancer has a thread-safety problem, increasing the current index but the lock is read lock. This may cause the result we get from the round-robin balancer may not be what we want.

Reference: https://github.com/zehuamama/balancer/blob/6e8ef295bbbc76b6815d9f297b26352aeaf37fd4/balancer/round_robin.go#L35C5-L35C5

[biningo]

I think could use the Go atomic package to slove this ploblem.

[hyman-m]

I think could use the Go atomic package to slove this ploblem.

嗯，非常感谢，我疏忽这个问题，支持使用原子命令，你可以提个 pr，我帮你合入