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. - 

{
        LOG_TRACE(log, "Found zookeper locks for {}", zookeeper_part_uniq_node);
        return false;
    }
    zookeeper->tryRemove(zookeeper_part_uniq_node);
    
    children.clear();
    zookeeper->tryGetChildren(zookeeper_part_node, children);
    if (children.empty())
        
        zookeeper->tryRemove(zookeeper_part_node);
    return true;
}
bool StorageReplicatedMergeTree::tryToFetchIfShared(
    const IMergeTreeDataPart & part,
    const DiskPtr & disk,
    const String & path)
{
    const auto data_settings = getSettings();
    if (!data_settings->allow_s3_zero_copy_replication)
        return false;
    if (disk->getType() != DB::DiskType::Type::S3)
        return false;
    String replica = getSharedDataReplica(part);
    
    if (replica.empty())
        return false;
    return executeFetchShared(replica, part.name, disk, path);
}
String StorageReplicatedMergeTree::getSharedDataReplica(