Replace hyper with a lighterweight framework

Hyper accounts for close to half of our external dependencies. We need to start weeding those out as much as possible or at least get a good control over them (see #2026).

Suggestions as base, both with far less dependencies:

https://github.com/tomaka/rouille https://github.com/steveklabnik/simple-server (would have to add TLS support)

Note that we could potentially create a fork of those and replace some of their dependencies with ours (same log framework, serde, etc). I think @hashmap and @garyyu have looked a bit at hyper replacements, any feedback?