title: When to use ExternallyControlledLifetimeManager?
body: <p>I was going through different LifetimeManagers available in Unity and was wondering when will we use ExternallyControlledLifetimeManager? Can somebody give me an real life example? The MSDN doc says "A LifetimeManager that holds a <em>weak reference</em> to it's managed instance".. </p>
  
  <p>How can the developer manage the instance and when should he go for such a lifetime manager? Your thoughts on how and when you used it would be great for me and others to understand.
  Thanks!</p>