Unify the "split client" package for BR and lightning

[lance6716]

Enhancement

now BR and lightning have separate functions to do one thing. Better to unify them

And the final goal is that lightning can use BR's new split&scatter strategy, which is first split to about #store, scatter them, and perform rest split to satisfy total region count

[okJiang]

Can you tell me which modules still need to be unified for this issue? @lance6716

[lance6716]

Can you tell me which modules still need to be unified for this issue? @lance6716

Do you mean why this issue is not finished? Reasons:

1. Currently we don't have enough test clsuter to verify it
2. I'm considering if the "split client" should have built-in coarse-grained + finer-grained two level split, don't let caller bother it.