Make data records optional with the kafka message frame schema

[zhouzhuojie]

Make data records optional and adopt the kafkaMessageFrame schema.

non-encrypted

{
  "payload": "{\"evalContext\":{\"enableDebug\":true,\"entityContext\":{\"state\":\"NY\"},\"entityID\":\"127\",\"entityType\":\"user\",\"flagID\":2},\"evalDebugLog\":{\"segmentDebugLogs\":[{\"msg\":\"constraint not match. constraint: state == \\\"CA\\\", entity_context: map[state:NY].\",\"segmentID\":2},{\"msg\":\"matched all constraints. rollout yes. {BucketNum:807 DistributionArray:{VariantIDs:[4 7] PercentsAccumulated:[500 1000]} VariantID:7 RolloutPercent:100}\",\"segmentID\":3}]},\"flagID\":2,\"segmentID\":3,\"timestamp\":\"2017-10-24T23:50:52Z\",\"variantAttachment\":{\"color\":\"dark-red\"},\"variantID\":7,\"variantKey\":\"red\"}",
  "encrypted": false
}

encrypted

{
  "payload": "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",
  "encrypted": true
}