S3 Multipart: Paused uploads hang indefinitely on resume

[yaegor]

When @uppy/aws-s3-multipart is used:

• start upload
• pause upload while some parts are being uploaded
• resume upload
  the parts are listed from the server and then the upload hangs in the "uploading" state and never continues, no attempts to re-upload missing parts are made.
  This is a regression, most probbaly brought in by uppy 2.0.0

[yaegor]

I've looked into the code more and lockedCandidatesForBatch cannot be just dropped as in #3342 as that can result in duplicate parts uploads. Seems like chunk's busy state can be set to true before preparing chunk for upload and since busy state is properly reset on upload errors, that should work.