Redis Cluster: delay state change when in the majority again.

As specified in the Redis Cluster specification, when a node can reach
the majority again after a period in which it was partitioend away with
the minorty of masters, wait some time before accepting queries, to
provide a reasonable amount of time for other nodes to upgrade its
configuration.

This lowers the probabilities of both a client and a master with not
updated configuration to rejoin the cluster at the same time, with a
stale master accepting writes.
Redis Cluster: handshake code refactoring + Gossip IP switch detection.

This commit makes it simple to start an handshake with a specific node
address, and uses this in order to detect a node IP change and start a
new handshake in order to fix the IP if possible.
Redis Cluster: reconfigure replication when master changes address.
Redis Cluster: remove no longer relevant comment.