Add sourceInfo to NotificationEvent (#3937)

This change adds information like host, port and user-agent of the
client whose request triggered an event notification.

E.g, if someone uploads an object to a bucket using mc. If notifications
were configured on that bucket, the host, port and user-agent of mc
would be sent as part of event notification data.

Sample output:
```
"source": {
          "host": "127.0.0.1",
          "port": "55808",
          "userAgent": "Minio (linux; amd64) minio-go/2.0.4 mc ..."
}
```

cmd/bucket-handlers.go

@@ -20,6 +20,7 @@ import (
 	"encoding/base64"
 	"encoding/xml"
 	"io"
+	"net"
 	"net/http"
 	"net/url"
 	"path"
@@ -322,6 +323,13 @@ func (api objectAPIHandlers) DeleteMultipleObjectsHandler(w http.ResponseWriter,
 	// Write success response.
 	writeSuccessResponseXML(w, encodedSuccessResponse)
 
+	// Get host and port from Request.RemoteAddr failing which
+	// fill them with empty strings.
+	host, port, err := net.SplitHostPort(r.RemoteAddr)
+	if err != nil {
+		host, port = "", ""
+	}
+
 	// Notify deleted event for objects.
 	for _, dobj := range deletedObjects {
 		eventNotify(eventData{
@@ -331,6 +339,9 @@ func (api objectAPIHandlers) DeleteMultipleObjectsHandler(w http.ResponseWriter,
 				Name: dobj.ObjectName,
 			},
 			ReqParams: extractReqParams(r),
+			UserAgent: r.UserAgent(),
+			Host:      host,
+			Port:      port,
 		})
 	}
 }
@@ -520,12 +531,21 @@ func (api objectAPIHandlers) PostPolicyBucketHandler(w http.ResponseWriter, r *h
 	w.Header().Set("ETag", `"`+objInfo.MD5Sum+`"`)
 	w.Header().Set("Location", getObjectLocation(bucket, object))
 
+	// Get host and port from Request.RemoteAddr.
+	host, port, err := net.SplitHostPort(r.RemoteAddr)
+	if err != nil {
+		host, port = "", ""
+	}
+
 	// Notify object created event.
 	defer eventNotify(eventData{
 		Type:      ObjectCreatedPost,
 		Bucket:    objInfo.Bucket,
 		ObjInfo:   objInfo,
 		ReqParams: extractReqParams(r),
+		UserAgent: r.UserAgent(),
+		Host:      host,
+		Port:      port,
 	})
 
 	if successRedirect != "" {