Symptoms: all threads in BackgroundScheduledPool busy, lot of tasks in replication queue, none of the tasks is executing. 

![Screenshot from 2021-04-16 16-21-44_](https://user-images.githubusercontent.com/1549571/115193796-e767d600-a0ec-11eb-887c-5c9c2412e50c.png)

Related to #16895

Possible workarounds (poor options): increasing the background_schedule_pool_size to huge values, or restart server every time when that happens etc. - 

StorageReplicatedMergeTree::allocateBlockNumber(
    const String & partition_id, const zkutil::ZooKeeperPtr & zookeeper, const String & zookeeper_block_id_path, const String & zookeeper_path_prefix) const
{
    String zookeeper_table_path;
    if (zookeeper_path_prefix.empty())
        zookeeper_table_path = zookeeper_path;
    else
        zookeeper_table_path = zookeeper_path_prefix;
    
    Coordination::Requests deduplication_check_ops;
    if (!zookeeper_block_id_path.empty())
    {
        deduplication_check_ops.emplace_back(zkutil::makeCreateRequest(zookeeper_block_id_path, "", zkutil::CreateMode::Persistent));
        deduplication_check_ops.emplace_back(zkutil::makeRemoveRequest(zookeeper_block_id_path, -1));
    }
    String block_numbers_path = fs::path(zookeeper_table_path) / "block_numbers";
    String partition_path = fs::path(block_numbers_path) / partition_id;
    if (!existsNodeCached(partition_path))
    {
        Coordination::Requests ops;