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/api-datatypes.go

@@ -20,6 +20,11 @@ import (
 	"encoding/xml"
 )
 
+const (
+	// Response request id.
+	responseRequestIDKey = "x-amz-request-id"
+)
+
 // ObjectIdentifier carries key name for the object to delete.
 type ObjectIdentifier struct {
 	ObjectName string `xml:"Key"`