Upgrade mio to verion 0.8.*

[sbosnick]

The crate mio dependency is version 0.6.* but the current version is 0.8.*. Upgrading mio will assist in being ready for verion 1.0.*.

Version 0.8.* has some breaking changes, but also includes a UnixStream type that has a try_io method. The upgrade to 0.8.* allows us to use this version of UnixStream instead of implementing Evented (which has been renamed Source) on top of fd-queue::net::UnixStream. We can use BiQueue and the mio UnixStream to implement fd-queue::mio::UnixStream and then separate out the mio-fd and net-fd features.