Optimize in-memory object size

Get the Java Object Layout tool from [1] and

• Measure the size of objects which are frequently allocated in memory, e.g.
  • Message
  • Event
  • Header subclasses (TpHeader, UNICAST3$Header, STABLE$Header, NakaAckHeader3)

Try to remove fields which are not absolutely needed and / or can be moved into the superclass.
E.g.: NakAckHeader2.sender might get removed: can't we determine the sender from the message ? (Maybe not, if sender is the original sender, not the sender of the message)

[1] https://github.com/Sanne/java-object-layout