No download limit - unused option MaxReadBeforeTruncate

gowarc has an option `MaxReadBeforeTruncate` with a default limit of 1GB. The problem is that this option isn’t used anywhere in gowarc and Zeno. Also, there was never such an option in `net/http`.

https://github.com/internetarchive/gowarc/blob/master/client.go#L139

This is a critical issue. A large target URL will OOM Zeno because when you use net/http to download a URL and you invoke `io.ReadAll()` it reads all in memory.