crimson/os/seastore: add linked tree nodes

Linked tree nodes encapsulate all logics about linking parent child
nodes with in-memory pointers.

There are three kinds of roles for linked tree nodes:

1. ParentNode: any nodes that have child nodes should be of this role,
   for example: (LBA/Backref)InternalNode, LBALeafNode.
2. ChildNode: any nodes that have parent nodes should be of this role,
   for example: (LBA/Backref)(Internal/Leaf)Node.
3. RootNode: any nodes that may be the root of a tree should be of this
   role.

Signed-off-by: Xuehan Xu <[email protected]>

Author: xxhdx1985126