This commit updates to the most recent versions (released today) of the various
Tokio libraries in use. Namely the `tokio_core::io` module has now been
deprecated in favor of an external `tokio-io` crate. This commit pulls in that
crate and uses the `AsyncRead + AsyncWrite` abstraction instead of `Io` from
tokio-core.
BREAKING CHANGE: Any external types that were using that had implemented `Io` will need to
implement `AsyncRead + AsyncWrite` from tokio_io.