Topology snapshots contain only mutation of current CDC generation data but don't
contain any previous or future generations. If new a generation of data is being
broadcasted but hasn't been entirely applied yet, the applied part won't be sent
in a snapshot. In this scenario, new or delayed nodes can never get the applied part.
Send entire cdc_generations_v3 table in the snapshot to resolve this problem.
As a follow-up, a mechanism to remove old CDC generations will be introduced.
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