Verison: Akka.NET v1.3.11

When I originally implemented CoordinatedShutdown, I left this little "TODO" in the source because .NET Standard 1.6 didn't expose an easy way for me to hook into process shutdown events, so I thought at least:

https://github.com/akkadotnet/akka.net/blob/dev/src/core/Akka/Actor/CoordinatedShutdown.cs#L682

Found this issue while working on petabridge/Cluster.WebCrawler#7

So, is there a way to implement this in .NET Standard 1.6 today? Or do we need to look at doing this within the confines of .NET Standard 2.0+?