GC: correctly collect TSOs

[hsyl20]

Currently the GC uses all the TSOs as roots (even the completed ones).

• [ ] Only use active TSOs as roots
• [ ] Don't forget to evacuate the link field on the active TSOs
• [ ] Blocked TSOs that are garbage-collected should be resurrected with the appropriate exception (BlockedIndefinitelyOnMVar/STM or NonTermination)

[Yveling23]

@hsyl20 #272 Duplicate of

• [ ] #641