TCP listener isn't closed in Tokio 0.2 on Windows

[Xanewok]

When migrating to Tokio 0.2 (in #603) we encountered an issue where the underlying OS socket wasn't closed on Windows (using mio v0.6) when shutting down Hyper server.

Because of this, the OS socket left open performs the TCP handshake but is incapable of doing any progress since the server has already quit. If a client attempts to connect, the connection itself will succeed but any further attempts to progress will block in mio.

For now, we explicitly close the OS socket when the Hyper server has quit but we would like to remove the workaround once we migrate to Tokio 1.0 (which has Windows backend overhauled in its mio v0.7 dependency).

Ref: #603 (comment)