Add kafka data logging

[zhouzhuojie]

Add async kafka data logging.

tested with kt consume.

curl --request POST \
  --url http://localhost:18000/api/v1/evaluation \
  --header 'content-type: application/json' \
  --data '{\n	"entityID": "123",\n	"entityType": "report",\n	"entityContext": {\n		"dl_state": "NY",\n		"doc_state": "NY"\n	},\n	"flagID": 2,\n	"enableDebug": true\n}'

produces

{
  "partition": 0,
  "offset": 14,
  "key": "123",
  "value": "{\"evalContext\":{\"enableDebug\":true,\"entityContext\":{\"dl_state\":\"NY\",\"doc_state\":\"NY\"},\"entityID\":\"123\",\"entityType\":\"report\",\"flagID\":2},\"evalDebugLog\":{\"segmentDebugLogs\":[{\"msg\":\"constraint not match. constraint: dl_state == \\\"CA\\\", entity_context: map[dl_state:NY doc_state:NY].\",\"segmentID\":2},{\"msg\":\"matched all constraints. rollout yes. {BucketNum:94 DistributionArray:{VariantIDs:[3 4] PercentsAccumulated:[300 1000]} VariantID:3 RolloutPercent:50}\",\"segmentID\":3}]},\"flagID\":2,\"segmentID\":3,\"timestamp\":\"2017-10-16T21:55:21Z\",\"variantID\":3}",
  "timestamp": "1969-12-31T15:59:59.999-08:00"
}