issue: #33583
the old policy permit datanode has at most 2 more channels than other
datanode. so if milvus has 2 datanode and 2 channels, both 2 channels
will be assign to 1 datanode, left another datanode empty.
This PR refine the balance policy to solve channel unbalance on datanode
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Signed-off-by: Wei Liu <wei.liu@zilliz.com>