on tablet split/merge finalization, generate a new CDC timestamp and
stream set for the table with a new stream for each tablet in the new
tablet map, in order to maintain synchronization of the CDC streams with
the tablets.
We pick a new timestamp for the streams with a small delay into the
future so that all nodes can learn about the new streams in time, in the
same way it's done for vnodes.
the new timestamp and streams are published by adding a mutation to the
cdc_streams_history table that contains the timestamp and the sets of
closed and opened streams from the current timestamp.
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