Upload locks on S3

[Acconut]

According to https://www.morling.dev/blog/leader-election-with-s3-conditional-writes/, it is now possible to implement a distributed lock on AWS S3 using conditional writes. Similar to how locks can be implemented on top of GCS: tus/tus-node-server#616

We should consider adding this capability to tusd, so users uploading to AWS S3 do not have to set up an additional locking mechanism. Of course, we have to consider S3-compatible interfaces as well, that do not support conditional writes (yet). Maybe we should provide an option to select the used locking implementation in the CLI.