Bugfix: Switch back to using the high-level minio-go API for s3

For the s3 backend we're back to using the high-level API the s3 client library
for uploading data, a few users reported dropped connections (which the library
will automatically retry now).

https://github.com/restic/restic/issues/1013
https://github.com/restic/restic/issues/1023
https://github.com/restic/restic/pull/1025
