Append an object to the linked list.

        Args:
            future (PlasmaObjectFuture): A PlasmaObjectFuture instance.

def append(self, future):
        """Append an object to the linked list.

        Args:
            future (PlasmaObjectFuture): A PlasmaObjectFuture instance.
        """
        future.prev = self.tail
        if self.tail is None:
            assert self.head is None
            self.head = future
        else:
            self.tail.next = future
        self.tail = future
        # Once done, it will be removed from the list.
        future.add_done_callback(self.remove)