api: Add new ListenBucketNotificationHandler. (#2336)

This API is precursor before implementing `minio lambda` and `mc` continous replication.

This new api is an extention to BucketNofication APIs.

// Request
```
GET /bucket?notificationARN=arn:minio:lambda:us-east-1:10:minio HTTP/1.1
...
...
```

// Response
```

{"Records": ...}
...
...
...
{"Records": ...}
```

config-old.go

@@ -146,6 +146,7 @@ func loadConfigV3() (*configV3, error) {
 	return c, nil
 }
 
+// logger type representing version '4' logger config.
 type loggerV4 struct {
 	Console struct {
 		Enable bool   `json:"enable"`
@@ -195,3 +196,81 @@ func loadConfigV4() (*configV4, error) {
 	}
 	return c, nil
 }
+
+// logger type representing version '5' logger config.
+type loggerV5 struct {
+	Console struct {
+		Enable bool   `json:"enable"`
+		Level  string `json:"level"`
+	} `json:"console"`
+	File struct {
+		Enable   bool   `json:"enable"`
+		Filename string `json:"fileName"`
+		Level    string `json:"level"`
+	} `json:"file"`
+	Syslog struct {
+		Enable bool   `json:"enable"`
+		Addr   string `json:"address"`
+		Level  string `json:"level"`
+	} `json:"syslog"`
+	AMQP struct {
+		Enable       bool   `json:"enable"`
+		Level        string `json:"level"`
+		URL          string `json:"url"`
+		Exchange     string `json:"exchange"`
+		RoutingKey   string `json:"routineKey"`
+		ExchangeType string `json:"exchangeType"`
+		Mandatory    bool   `json:"mandatory"`
+		Immediate    bool   `json:"immediate"`
+		Durable      bool   `json:"durable"`
+		Internal     bool   `json:"internal"`
+		NoWait       bool   `json:"noWait"`
+		AutoDeleted  bool   `json:"autoDeleted"`
+	} `json:"amqp"`
+	ElasticSearch struct {
+		Enable bool   `json:"enable"`
+		Level  string `json:"level"`
+		URL    string `json:"url"`
+		Index  string `json:"index"`
+	} `json:"elasticsearch"`
+	Redis struct {
+		Enable   bool   `json:"enable"`
+		Level    string `json:"level"`
+		Addr     string `json:"address"`
+		Password string `json:"password"`
+		Key      string `json:"key"`
+	} `json:"redis"`
+}
+
+// configV5 server configuration version '5'.
+type configV5 struct {
+	Version string `json:"version"`
+
+	// S3 API configuration.
+	Credential credential `json:"credential"`
+	Region     string     `json:"region"`
+
+	// Additional error logging configuration.
+	Logger loggerV5 `json:"logger"`
+}
+
+// loadConfigV5 load config version '5'.
+func loadConfigV5() (*configV5, error) {
+	configFile, err := getConfigFile()
+	if err != nil {
+		return nil, err
+	}
+	if _, err = os.Stat(configFile); err != nil {
+		return nil, err
+	}
+	c := &configV5{}
+	c.Version = "5"
+	qc, err := quick.New(c)
+	if err != nil {
+		return nil, err
+	}
+	if err := qc.Load(configFile); err != nil {
+		return nil, err
+	}
+	return c, nil
+}