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/server-startup-msg_test.go

@@ -94,3 +94,39 @@ func TestCertificateNotExpired(t *testing.T) {
 		t.Fatalf("Expected empty message was: %s", msg)
 	}
 }
+
+// Test printing server common message.
+func TestPrintServerCommonMessage(t *testing.T) {
+	root, err := newTestConfig("us-east-1")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer removeAll(root)
+
+	apiEndpoints := []string{"127.0.0.1:9000"}
+	printServerCommonMsg(apiEndpoints)
+}
+
+// Tests print cli access message.
+func TestPrintCLIAccessMsg(t *testing.T) {
+	root, err := newTestConfig("us-east-1")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer removeAll(root)
+
+	apiEndpoints := []string{"127.0.0.1:9000"}
+	printCLIAccessMsg(apiEndpoints[0])
+}
+
+// Test print startup message.
+func TestPrintStartupMessage(t *testing.T) {
+	root, err := newTestConfig("us-east-1")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer removeAll(root)
+
+	apiEndpoints := []string{"127.0.0.1:9000"}
+	printStartupMessage(apiEndpoints)
+}