Add "validatorFor" to RecordTransformBuffer

[enspandi]

As otherwise it will ignore custom validators.

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I also tried passing in a custom transformBuffer to the indexeddb source settings as mentioned in #868 - but it is not passed forward to the indexeddb cache settings....

[enspandi]

@dgeb I should note that custom validator works with Indexed db source if

settings.defaultTransformOptions = { useBuffer: true };

is not set.

It seems to run the validation process of the indexeddb cache class which uses the correct validatorFor function. But in case of useBuffer: true it will create a new transform buffer class that does not have it.