actually fix the memleak around smpi's process_data

actually fix the memleak around smpi's process_data

- Call Actor::onDestruction from the right location
- Have SMPI only destroy the process data of its own processes (not of MSG ones)
- Make sure that the process created with MSG_process_attach() have
  the right refcount since we now use a smart pointer on it: we don't
  want it to get freed too early.