Adding some graceful shutdown handling to ProtoActorLifecycleHost

[jstnlef]

Description

Since the proto.actor cluster has the ability to shut itself down when other nodes in the system believe that it should be shutdown, I believe it makes sense for this to also trigger a process shutdown. Perhaps this should be made optional?

Purpose

This pull request is a:

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)