Add batch evaluation

[zhouzhuojie]

Batch evaluation.

curl --request POST \
  --url http://localhost:18000/api/v1/evaluation/batch \
  --header 'content-type: application/json' \
  --data '{\n  "entities": [\n    {\n      "entityID": "123",\n      "entityType": "user",\n      "entityContext": {\n				"state": "CA"\n			}\n    }\n  ],\n  "enableDebug": true,\n  "flagIDs": [\n    4,2\n  ]\n}'

{
	"evaluationResults": [
		{
			"evalContext": {
				"enableDebug": true,
				"entityContext": {
					"state": "CA"
				},
				"entityID": "123",
				"entityType": "user",
				"flagID": 4
			},
			"evalDebugLog": {
				"msg": "flagID 4 has no segments",
				"segmentDebugLogs": null
			},
			"flagID": 4,
			"segmentID": null,
			"timestamp": "2017-10-20T18:57:27Z",
			"variantAttachment": null,
			"variantID": null,
			"variantKey": null
		},
		{
			"evalContext": {
				"enableDebug": true,
				"entityContext": {
					"state": "CA"
				},
				"entityID": "123",
				"entityType": "user",
				"flagID": 2
			},
			"evalDebugLog": {
				"segmentDebugLogs": [
					{
						"msg": "matched all constraints. rollout yes. {BucketNum:94 DistributionArray:{VariantIDs:[3] PercentsAccumulated:[1000]} VariantID:3 RolloutPercent:50}",
						"segmentID": 2
					}
				]
			},
			"flagID": 2,
			"segmentID": 2,
			"timestamp": "2017-10-20T18:57:27Z",
			"variantAttachment": {
				"color": "super-green"
			},
			"variantID": 3,
			"variantKey": "green"
		}
	]
}