event: Enhance event message struct to provide origin server. (#3557)

`principalId` i.e user identity is kept as AccessKey in
accordance with S3 spec.

Additionally responseElements{} are added starting with

`x-amz-request-id` is a hexadecimal of the event time itself in nanosecs.
`x-minio-origin-server` - points to the server generating the event.

Fixes #3556

cmd/bucket-policy.go

@@ -24,6 +24,12 @@ import (
 	"sync"
 )
 
+const (
+	// Static prefix to be used while constructing bucket ARN.
+	// refer to S3 docs for more info.
+	bucketARNPrefix = "arn:" + eventSource + ":::"
+)
+
 // Variable represents bucket policies in memory.
 var globalBucketPolicies *bucketPolicies