Server runs an HTTP server with an S3-like API for PFS. This allows you to use s3 clients to acccess PFS contents.
This returns an `http.Server` instance. It is the responsibility of the caller to start the returned server. It's possible for the caller to gracefully shutdown the server if desired; see the `http` package for details.
Note: server errors are redirected to logrus' standard log writer. The log writer is never closed. This should not be a problem with logrus' default configuration, which just writes to stdio. But if the standard logger is overwritten (e.g. to write to a socket), it's possible for this to cause problems.
Note: In `s3cmd`, you must set the access key and secret key, even though this API will ignore them - otherwise, you'll get an opaque config error: https://github.com/s3tools/s3cmd/issues/845#issuecomment-464885959