pub fn pruning_task(&mut self, poll_interval: Duration) -> MerkleTreePruningTask {
        let (pruning_handles_sender, pruning_handles) = oneshot::channel();
        self.pruning_handles_sender = pruning_handles_sender;
        MerkleTreePruningTask::new(pruning_handles, self.pool.clone(), poll_interval)
    }
Returns a task that can be used to prune the Merkle tree according to the pruning logs in Postgres.
This method should be called once; only the latest returned task will do any job, all previous ones
will terminate immediately.
core/node/metadata_calculator/src/lib.rs