When starting in legacy mode, a gossip event listener called persistent
feature enabler is registered. This listener marks a feature as enabled
when it notices, in gossip, that all nodes declare support for the
feature.
With raft-based topology, features are managed in group 0 instead and do
not rely on the persistent feature enabler at all. Make the listener
look at the raft_topology_change_enabled() method and prevent it from
enabling more features after that method starts returning true.
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