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-utils.go

@@ -0,0 +1,67 @@
+/*
+ * Minio Cloud Storage, (C) 2016, 2017 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package cmd
+
+import (
+	"fmt"
+	"net"
+	"net/http"
+)
+
+// getListenIPs - gets all the ips to listen on.
+func getListenIPs(serverAddr string) (hosts []string, port string, err error) {
+	var host string
+	host, port, err = net.SplitHostPort(serverAddr)
+	if err != nil {
+		return nil, port, fmt.Errorf("Unable to parse host address %s", err)
+	}
+	if host == "" {
+		var ipv4s []net.IP
+		ipv4s, err = getInterfaceIPv4s()
+		if err != nil {
+			return nil, port, fmt.Errorf("Unable reverse sort ips from hosts %s", err)
+		}
+		for _, ip := range ipv4s {
+			hosts = append(hosts, ip.String())
+		}
+		return hosts, port, nil
+	} // if host != "" {
+	// Proceed to append itself, since user requested a specific endpoint.
+	hosts = append(hosts, host)
+	return hosts, port, nil
+}
+
+// Finalizes the API endpoints based on the host list and port.
+func finalizeAPIEndpoints(tls bool, apiServer *http.Server) (endPoints []string) {
+	// Verify current scheme.
+	scheme := "http"
+	if tls {
+		scheme = "https"
+	}
+
+	// Get list of listen ips and port.
+	hosts, port, err := getListenIPs(apiServer.Addr)
+	fatalIf(err, "Unable to get list of ips to listen on")
+
+	// Construct proper endpoints.
+	for _, host := range hosts {
+		endPoints = append(endPoints, fmt.Sprintf("%s://%s:%s", scheme, host, port))
+	}
+
+	// Success.
+	return endPoints
+}