[fileStorage] Add an API to remove multiple files at once

[tats-u]

https://docs.aws.amazon.com/AmazonS3/latest/userguide/delete-multiple-objects.html

It would be better if fileStorage can remove multiple keys at once (at one HTTP request for network storages).

It is a little bit pain in the neck and inefficient to list every file and issue .remove per one file.

S3 seems to limit the number of files that can be removed at one request.