Make TLS backend configurable
At least on OpenBSD, native-tls uses libressl as the TLS backend. Unfortunately, it often has problems: https://twitter.com/vext01/status/1139106175608328193
Since we only use TLS via Hyper and Hyper supports multiple TLS backends, perhaps it makes sense to support other backends. In particular, there is https://docs.rs/hyper-rustls/0.16.1/hyper_rustls/ , which allows Hyper to use Rustls.