Memory management in BusPacket

- Changed to a unique_ptr
- correction in pre hook: changed pointer reference to a const
  reference. That way the pointer is not mutable from within a pre-hook
  plugin. Reasons for this change: If creation and deletion of objects
  must be handled in MBus application itself, this restriction can
  prevent harmful errors. Up to now, packets are created and deleted in
  different places, which will not cause any trouble, as long as
  libraries and bus are compiled the same way. If we ever restrict this
  further, the pre-hook may not allow to exchange objects!