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This will be a silent breaking change since it flips the behavior around. 

Existing mirrormakers will be unaffected for any topics they are currently consuming since they already have a saved offset.

Mirrormakers that start consuming topics for which they don't have a saved offset will start replicating the partitions from the beginning, rather than from the partition's current highwater mark. If the mirrormakers mirrormaker starts consuming a very large partition/topic, it will replicate far more data than expected. This has relatively low probability since most of these topics are going to be newly-created topics anyway, so most of the time starting from the earliest simply prevents skipping the first few seconds/minutes of data written to the topic.

Existing mirrormakers will be unaffected for any topics they are currently consuming since they already have a saved offset.

Since MirrorMaker 2.0 already behaves this way, this change will make future migrations from MM1 to MM2 easier for folks since the behavior will stop changing between them.

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