Cluster: always use safe iteartors to iterate server.cluster->nodes.
Cluster: don't save HANDSHAKE nodes in nodes.conf.
Cluster: CLUSTER SAVECONFIG command added.
redis-cli: fix big keys search when the key no longer exist.

The code freed a reply object that was never created, resulting in a
segfault every time randomkey returned a key that was deleted before we
queried it for size.
Cluster: free HANDSHAKE nodes after node_timeout.

Handshake nodes should turn into normal nodes or be freed in a
reasonable amount of time, otherwise they'll keep accumulating if the
address they are associated with is not reachable for some reason.
Cluster: don't add an handshake node for the same ip:port pair multiple times.