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": ...}
```

handler-utils.go

@@ -22,6 +22,7 @@ import (
 	"mime/multipart"
 	"net/http"
 	"strings"
+	"time"
 )
 
 // Validates location constraint in PutBucket request body.
@@ -129,3 +130,16 @@ func extractPostPolicyFormValues(reader *multipart.Reader) (filePart io.Reader,
 	}
 	return filePart, fileName, formValues, nil
 }
+
+// Send whitespace character, once every 5secs, until CompleteMultipartUpload is done.
+// CompleteMultipartUpload method of the object layer indicates that it's done via doneCh
+func sendWhiteSpaceChars(w http.ResponseWriter, doneCh <-chan struct{}) {
+	for {
+		select {
+		case <-time.After(5 * time.Second):
+			w.Write([]byte(" "))
+		case <-doneCh:
+			return
+		}
+	}
+}