Cluster: fix rebalancing to always empty nodes.
Cluster: fix rebalancing to always empty nodes.

Because of rounding error even with weight=0 sometimes a node was left
with an assigned slot.

Close #3001.

4396/redis,pedigree/redis,janekmi/redis,4396/redis,pedigree/redis,4396/redis,janekmi/redis,janekmi/redis,xuguruogu/redis,pedigree/redis,xuguruogu/redis,4396/redis,xuguruogu/redis,janekmi/redis,pedigree/redis,xuguruogu/redis