Cluster: don't migrate to a master that never had slaves.

Replica migration algorithm modified so that slaves never try to migrate
to masters that were never configured to have slaves in the past.
We want the algorithm to take care of masters that remained without
*working* slaves, but that used to have slaves according to the cluster
configuration.
Cluster test: new unit 05, slave selection.
Example redis.conf: improve slaveof description.
Cluster test: unit 06, slaves with stale data can't failover.
Cluster test: 03 unit speedup, don't send WAIT to slaves.
Cluster test: init test, be patient during restarts.
Cluster test: node reachability condition fixed.
Fix spelling of "stand alone" in ascii art

Also unified the logic to match `genRedisInfoString`
Cluster test: unit 06 cluster creation comment fixed.
Cluster test: unit 07, replicas migration.
* fixed doc URL for keyspace events
Fail SYNC if background save child aborted due to a signal.