title: ReplaySubject: Only replay for a single IObservable
body: <p>I'm trying to find a way to replay messages from a hot IObservable, but only do so for a single IObserver.  And do so using minimal memory.</p>
  
  <p>If I use ReplaySubject, then I assume it keeps a copy of each item until the IObservable is disposed of.  I actually only want it to keep a copy of each item until the IObserver attaches itself.  After that point, it can act as if the ReplaySubject weren't even there.</p>