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": ...}
```

bucket-policy.go

@@ -30,15 +30,6 @@ func getOldBucketsConfigPath() (string, error) {
 	return filepath.Join(configPath, "buckets"), nil
 }
 
-// getBucketConfigPath - get bucket config path.
-func getOldBucketConfigPath(bucket string) (string, error) {
-	bucketsConfigPath, err := getOldBucketsConfigPath()
-	if err != nil {
-		return "", err
-	}
-	return filepath.Join(bucketsConfigPath, bucket), nil
-}
-
 // readBucketPolicy - read bucket policy.
 func readBucketPolicy(api objectAPIHandlers, bucket string) ([]byte, error) {
 	// Verify bucket is valid.