Change way Mailboxes are create, stored, and destroyed

Change way Mailboxes are create, stored, and destroyed

- keep the global map in EngineImpl (not as a static global in
  MailboxImpl.cpp)
- Delete mailboxes in ~EngineImpl() and get rid of SIMIX_mailbox_exit
- replace MailboxImpl::by_name_or_null and
  MailboxImpl::by_name_or_create by Engine::mailbox_by_name_or_create
  + better match with what is done for hosts, links, and actors
  + Mailbox::by_name cause two searchs in the map. One to check if
    name already points to a mailbox (by_name_or_null) and if not
    search again before creating a new mailbox. As there is no
    Mailbox::by_name_or_null, just keep the latter.
- Revalidate a bunch of tests (message ordering mostly)