Update SegmentFetcher to use Validator

[jefft0]

The SegmentFetcher uses a security v1 validatorKeyChain to validate packets (or null to not validate). Since we removed security v1 support, we need to update SegmentFetcher and the related PipelineCubic and PipelineFixed to use a security v2 Validator. (At the moment, we don't have applications which use a SegmentFetcher, and the examples use null for the validator, so it is not an immediate concern. This issue is here to keep a note.)