* Update 'tokio', 'tokio-rustls', and 'hyper'.
* Remove unused dependencies on some `futures-*` crates.
* Rework 'spawn_on', which is now 'serve'.
* Simplify Ctrl-C handling.
Use I/O traits and types from 'tokio-io' as much as possible.
A few adapters only exist in futures-io-preview and use
futures-tokio-compat as a bridge for now.