In accordance with tokio-rs/tokio-rfcs#3, the executor functionality of
Tokio is being removed and will be relocated into futures-rs as a
"current thread" executor.
This PR removes task execution from the code base. As a temporary
mesure, all examples and tests are switched to using CpuPool.
Depends on #19.
* Move to `std::net` as it's all purely UDP related
* Rename to `UdpCodec` and `UdpFramed` to give a consistent `Udp` prefix
* Add `RecvDgram`, rename `SendDGram` to `SendDgram`
* Touch up some style here and there