Always use raft address map to obtain the IP addresses
of upgrade peers. Right now the map is populated
from Raft configuration, so it's an equivalent transformation,
but in the future raft address map will be populated from other sources:
discovery and gossip, hence the logic of upgrade will change as well.
Do not proceed with the upgrade if an address is
missing from the map, since it means we failed to contact a raft member.
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