Speed up messageNew by avoiding _messageParent
Speed up messageNew by avoiding _messageParent

* Initial insertion now includes root (and submessage data if root),
  which helps avoid jitter in since/live views
* _submessagesChanged called directly from mesageNew now
* Key operation needed by messageNew and _messageParent
  extracted to _messageAddChild

edemaine/coauthor,edemaine/coauthor