This patch disables various Unix-specific platform features that are
not enabled on Fuchsia. It also updates the mio version to 0.6.10,
which is the first release that supports Fuchsia.
* 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