Cluster: fix setting nodes slaveof pointer to NULL on node release.
Cluster: fix setting nodes slaveof pointer to NULL on node release.

With this commit we preserve the list of nodes that have .slaveof set
to the node, even when the node is turned into a slave, and make sure to
fix the .slaveof pointers to NULL when a node is freed from memory,
regardless of the fact it's a slave or a master.

Basically we try to remember the logical master in the current
configuration even if the logical master advertised it as a slave
already. However we still remember the associations, so that when a node
is freed we can fix them.

This should fix issue #3002.

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