Gracefully close connections on server shutdown

[djones6]

See IBM-Swift/SwiftyRequest#69

The analysis of an issue involving failures from a shared URLSession when the server is repeatedly stopped/started concluded that the issue relates to when the HTTPServer is stopped, and any open connections are abruptly closed. Curl doesn't seem to handle this well.

This was resolved for Kitura-NIO under https://github.com/IBM-Swift/Kitura-NIO/issues/216 where we concluded that connections should be closed gracefully on server stop. The same should be applied to Kitura-net.