Fix replicas migration by adding a new flag.
Some time ago I broken replicas migration (reported in #2924). The idea was to prevent masters without replicas from getting replicas because of replica migration, I remember it to create issues with tests, but there is no clue in the commit message about why it was so undesirable. However my patch as a side effect totally ruined the concept of replicas migration since we want it to work also for instances that, technically, never had slaves in the past: promoted slaves. So now instead the ability to be targeted by replicas migration, is a new flag "migrate-to". It only applies to masters, and is set in the following two cases: 1. When a master gets a slave, it is set. 2. When a slave turns into a master because of fail over, it is set. This way replicas migration targets are only masters that used to have slaves, and slaves of masters (that used to have slaves... obviously) and are promoted. The new flag is only internal, and is never exposed in the output nor persisted in the nodes configuration, since all the information to handle it are implicit in the cluster configuration we already have.
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