Make mergeBatched protected instead of private.

[non]

There are reasons why particular implementations might want to override this method (for example, to support custom serialization strategies), so let's make it protected instead of private.

[johnynek]

👍

Thanks!

merge when green.

[johnynek]

looks like this fails mima, naturally...

So we can make the change at the next mima break, maybe when we release for scalding 0.18

[CLAassistant]

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