persistent_feature_enabler: attach to gossip only if not using raft

The enable_features_on_join function is now only called if the node does
not use topology over raft, and so the node will not react to changes in
gossip features.

In the future, support for switching to topology coordinator in runtime
will be added and the persistent feature enabler should disconnect
itself during the upgrade procedure. We don't have such procedure yet,
so a bunch of TODOs is added instead.

service/storage_service.cc

@@ -2397,7 +2397,10 @@ future<> storage_service::join_token_ring(cdc::generation_service& cdc_gen_servi
     });
     _listeners.emplace_back(make_lw_shared(std::move(schema_change_announce)));
     co_await _gossiper.wait_for_gossip_to_settle();
-    co_await _feature_service.enable_features_on_join(_gossiper, _sys_ks.local());
+    // TODO: Look at the group 0 upgrade state and use it to decide whether to attach or not
+    if (!_raft_topology_change_enabled) {
+        co_await _feature_service.enable_features_on_join(_gossiper, _sys_ks.local());
+    }
 
     set_mode(mode::JOINING);