[Enhancement]Smooth Round Robin selection #2647

[kimmking]

@jasonjoo2010 submit a new Smooth Round Robin for:

1. smooth for all providers
2. more better in concurrent cases

The Round Robin balancer is not smooth with weights, such as:

A:B:C = 1:2:3

Will be:
[A, B, B, C, C, C, A, B, B, C, C, C, A, B, B, C, C, C, ..........]

But we want it to be:
[A, B, C, B, C, C, A, B, C, B, C, C, A, B, C, B, C, C, ..........]

which has been discuss before for performance: #2578

Brief changelog
reimplement RoundRobinLoadBalance and its unit test.

More discussion here:
#2578

Also see:
#2647