Gensio 2.8.15

This release adds the ability to set the timeout waiting for output to clear. The default has not changed, but you can set a drain_timeout option on many gensios that will set the time to wait before dropping data on the floor and shutting down the connection. This will help in situations where a remote end crashes or a network partition happens.

This will be the last 2.8.x release with new features. It's going to maintenance, and I will be starting a 3.0.x series. This will remove the sergensio code that has been deprecated for a long time.