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.

yossigo/redis,zguangyu/redis,Sciumo/redis,Markgorden/redis,nnog/redis,ctripcorp/redis,Markgorden/redis,PKRoma/redis,nnog/redis,soveran/redis,OmarQunsul/graph-redis,spinlock/redis,arijitvt/redis,pmem/redis,soloestoy/redis,badboy/redis,dreamquster/redis,ton31337/redis,GitHubMota/redis,pmem/redis,soveran/redis,soveran/redis,oranagra/redis,ofirluzon/redis,dreamquster/redis,kensou97/redis,ton31337/redis,rouzier/redis,antirez/redis,neomantra/redis,hgl888/redis,rouzier/redis,h0x91b/redis,GitHubMota/redis,rogerchina/redis,PKRoma/redis,GitHubMota/redis,h0x91b/redis,Sciumo/redis,ofirluzon/redis,oranagra/redis,colstrom/redis,arijitvt/redis,ofirluzon/redis,charsyam/redis,antirez/redis,spinlock/redis,hgl888/redis,zguangyu/redis,OmarQunsul/graph-redis,sunheehnus/redis,shreesundara/redis,YongMan/redis,colstrom/redis,rogerchina/redis,GitHubMota/redis,soveran/redis,badboy/redis,h0x91b/redis,yossigo/redis,OmarQunsul/graph-redis,soloestoy/redis,neomantra/redis,ton31337/redis,pmem/redis,shreesundara/redis,ofirluzon/redis,yossigo/redis,ctripcorp/redis,kensou97/redis,kensou97/redis,oranagra/redis,nnog/redis,YongMan/redis,rouzier/redis,sunheehnus/redis,sunheehnus/redis,soloestoy/redis,YongMan/redis,antirez/redis,neomantra/redis,charsyam/redis,yossigo/redis,pmem/redis,dreamquster/redis,Sciumo/redis,spinlock/redis,Sciumo/redis,badboy/redis,rogerchina/redis,charsyam/redis,zguangyu/redis,soloestoy/redis,YongMan/redis,ofirluzon/redis,hgl888/redis,Markgorden/redis,shreesundara/redis,antirez/redis,neomantra/redis,PKRoma/redis,spinlock/redis,dreamquster/redis,OmarQunsul/graph-redis,sunheehnus/redis,PKRoma/redis,ctripcorp/redis,ctripcorp/redis,Markgorden/redis,arijitvt/redis,colstrom/redis,charsyam/redis,kensou97/redis,yossigo/redis,arijitvt/redis,ton31337/redis,soloestoy/redis,charsyam/redis,rouzier/redis,neomantra/redis,colstrom/redis,badboy/redis,oranagra/redis,h0x91b/redis,shreesundara/redis,rogerchina/redis,oranagra/redis,PKRoma/redis,hgl888/redis,zguangyu/redis,nnog/redis