Use multipart uploads for large file uploads

[clue]

The S3 upload API is limited to 5GB, so larger files have to be uploaded in chunks. Additionally, we may also chunk smaller files and concurrently upload multiple parts.

Refs #2 and https://developers.digitalocean.com/documentation/spaces/#begin-multi-part-upload

[clue]

See also clue/reactphp-buzz#138 for current timeout handling when uploading large files. For now, we might as well disable any timeouts in this library.