Rollback: use cas() to claim the closure in copyPart(), to match copy_tag()

Rollback: use cas() to claim the closure in copyPart(), to match copy_tag()

rolling back:

  * use cas() to claim the closure in copyPart(), to match copy_tag()
  * rename whitehole_spin to evac_collision, and update it properly

This introduced a new failure in parallel GC.  I'll rollback for now
until I've fixed it.