Remove references to the VM secondMemoryManager

* No one is allowed to use the secondMemoryManager except the GC and the VM.
* Thus, getSecondMemoryManager() is removed.
* GraphReplicator now takes a sourceMM, which is used
  to allocate temporary structures for doing the copy.
  The copy is always done to the (primary) memoryManager.
* We want not to depend on the existence of the secondMemoryManager
  as we might free its block between GCs or share the block between VMs.