Expose IApplicationShutdown interface from ApplicationHost

[davidfowl]

Related to aspnet/Hosting#23

// provided by root-most layer, akin to IApplicationEnvironment
// dependency-injected into hosting layer to subscribe to shutdownrequested token
// dependency-injected into file watcher or other low-level components
// which have a reason to request process shutdown
[AssemblyNeutral]
public interface IApplicationShutdown
{
  // called by subsystems that wish to initiate a graceful shutdown.
  // typical callers include file watcher
  // expect to return immediately.
  void RequestShutdown();

  // token triggered the first time RequestShutdown is called
  // typical observers include helios or self-host's "main" method
  // expected behavior is to return from "main" which will result in
  // the rest of the tear-down sequence occuring
  // including triggering IHttpApplicationLifecycle events
  // and ultimately process termination or recycling
  CancellationToken ShutdownRequested {get;}
}