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-headers_test.go

@@ -18,11 +18,12 @@ package cmd
 
 import (
 	"testing"
+	"time"
 )
 
 func TestNewRequestID(t *testing.T) {
 	// Ensure that it returns an alphanumeric result of length 16.
-	var id = mustGetRequestID()
+	var id = mustGetRequestID(time.Now().UTC())
 
 	if len(id) != 16 {
 		t.Fail()