Update StoreTreeNode methods

yang.data.api.schema.tree.StoreTreeNode.getChild() is forcing users to deal with optionals, where a lot of the time they would prefer to deal with a plain nullable.

Update StoreTreeNode so that it has three methods:

@Nullable TreeNode childByArg(PathArgument);

@NonNull TreeNode getChildByArg(PathArgument); // throws VerifyException

Optional<TreeNode> findChildByArg(PathArgument);