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This checkpoint file is also created by the kafka-storage tool in the __cluster-metadata-0  directory. Now, it will also contain the data that used to be contained in the bootstrap.checkpoint file.  It is important to note that the bootstrap records must be less than 8MB of total size, since that is the maximum batch size in bytes supported by KRaft.

Proposed Changes

Controller

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1. bootstrap.checkpoint exist with metadata records and the zero checkpoint doesn't exists - In this case the controller will behave as it does today. The controller will be able to identify this case because RaftClient.Listener#handleLoadSnapshot won't ask to load the zero snapshot.
2. bootstrap.checkpoint exist with metadata records and the zero checkpoint exists but doesn't contain any metadata records - In this case the controller will behave as it does today. The controller will be able to identify this case because RaftClient.Listener#handleLoadSnapshot will ask to load the zero checkpoint but it will be empty, no metadata records.
3. bootstrap.checkpoint doesn't exist and the zero checkpoint exists with metadata records - In this case the controller will use the zero checkpoint's metadata records to and write them to the log in a transaction or single atomic batch like the controller does pre-KIP-1170.

Test Plan

Describe in few sentences how the KIP will be tested. We are mostly interested in system tests (since unit-tests are specific to implementation details). How will we know that the implementation works as expected? How will we know nothing broke?

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