`cdc::generation_service::make_new_cdc_generation` would create a new
CDC generation and insert it into the `CDC_GENERATIONS_V2` table these
days. For Raft-based topology chnages we'll do the data insertion
somewhere else - in topology coordinator code. So extract the parts for
calculating the CDC generation to free-standing functions (these are
almost pure calculations, modulo accessing RNG).
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