This is the legacy struct that used to be emitted as part of a transactional produce and
contains the largest timestamp within the batch committed. Since it is just a timestamp it
cannot encode the fact that a sink has finished and deviates from upper frontier semantics.
Materialize no longer produces this record but it’s possible that we encounter this in topics
written by older versions. In those cases we convert it into upper semantics by stepping the
timestamp forward.
This struct is emitted as part of a transactional produce, and contains the upper frontier of
the batch committed. It is used to recover the frontier a sink needs to resume at.
Configuration of a topic created by ensure_kafka_topic
.