[coco] Default constructible DLinkedList::Node (#926)

[coco] Default constructible DLinkedList::Node (#926)

The current design of DLinkedList::Node enforces all of its descendent classes to
implement a consturctor that takes PtrLink, and passes it to its parent.

This restriction makes it difficult to introduce some intermediate
traists.

This commit revises DLinkedList::Node to remove this restriction.

The current design enforces only leaf classes to implement 'get' method.

Signed-off-by: Jonghyun Park <jh1302.park@samsung.com>