Those nodes will be kept in tablet replica sets for a while after node
replace is done, until the new replica is rebuilt. So we need to know
about those node's location (dc, rack) for two reasons:
1) algorithms which work with replica sets filter nodes based on
their location. For example materialized views code which pairs base
replicas with view replicas filters by datacenter first.
2) tablet scheduler needs to identify each node's location in order
to make decisions about new replica placement.
It's ok to not know the IP, and we don't keep it. Those nodes will not
be present in the IP-based replica sets, e.g. those returned by
get_natural_endpoints(), only in host_id-based replica
sets. storage_proxy request coordination is not affected.
Nodes in the left state are still not present in token ring, and not
considered to be members of the ring (datacanter endpoints excludes them).
In the future we could make the change even more transparent by only
loading locator::node* for those nodes and keeping node* in tablet
replica sets.
We load topology infromation only for left nodes which are actually
referenced by any tablet. To achieve that, topology loading code
queries system.tablet for the set of hosts. This set is then passed to
system.topology loading method which decides whether to load
replica_state for a left node or not.
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