Removes 500 public assets limitation and makes files uploads concurrent

[nunomaduro]

This pull request removes 400 public assets limitation and makes files uploads concurrent.

• The 400 public assets limitation is removed by applying pagination - using 400 elements - when reaching Vapor API. So, when asking the Vapor API for the store/copy signed URL, we lazy paginate over each 400 elements.
• Next, this pull request also contains code that makes files uploads concurrent, uploading streaming 10 files per S3 store object request.
• Finally, this pull request also provides store / copying feedback in real-time on the console.