Garbage collection progress delegate

[Craig Black]

> - start of gc collecting cycle
> - gc scans refb, which is null
> - gc calls your delegate
> - in your delegate refb = refa; refa = null;
> - your delegate ends, gc continues ('thinking' refb is null)
> - scans refa, which is now null also
> -> GC: Oh, no ref to obj, I can free it!

Hmmm.  I don't see this as a showstopper.  Just avoid assigning references 
when providing feedback.  (Use of the GC delegate would be for advanced 
users anyway.)

-Craig