After changing `left_token_ring` from a node state to a transition
state in scylladb/scylladb#17009, we do the same for
`rollback_to_normal`. `rollback_to_normal` was created as a node
state because `left_token_ring` was a node state.
This change will allow us to distinguish a failed removenode from
a failed decommission in the `rollback_to_normal` handler.
Currently, we use the same logic for both of them, so it's not
required. However, this might change, as it has happened with the
decommission and the failed bootstrap/replace in the
`left_token_ring` state (scylladb/scylladb#16797). We are making
this change now because it would be much harder after branching.
The change also simplifies the code in
`topology_coordinator:rollback_current_topology_op`.
Moving the `rollback_to_normal` handler from
`handle_node_transition` to `handle_topology_transition` created a
large diff. There is only one change - adding
`auto node = get_node_to_work_on(std::move(guard));`.
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