GetChunkserverChinas handle small clusters

Empty cluster, 5 cs, 3 replica, 10 threads. Ns will always choose the first three cs and leave the rest 2 behind.
空集群，5个cs，3副本，10个线程并发写时，计算buffer都是0，都会选择相同的3个cs，其余2个cs空闲

Start by locating the GetChunkserverChinas entry point and trace chunkserver selection for the reported empty-cluster case. Reproduce the scenario with 5 chunkservers, 3 replicas, and 10 concurrent writers; done should mean the selection no longer repeatedly chooses only the same three servers, while the intended balancing behavior is confirmed.

Written by the indexing model from the issue text.