The topology coordinator fiber is not started if a node starts in legacy
topology mode. We need to start the raft state monitor fiber after all
preconditions for starting upgrade to raft topology are met.
Add a fiber which is spawned only in legacy mode that will wait until:
- The schema-on-raft upgrade finishes,
- The SUPPORTS_CONSISTENT_CLUSTER_MANAGEMENT feature is enabled,
- The upgrade is triggered by the user.
and, after that, will spawn the raft state monitor fiber.
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