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. - 

else
    {
        ops.emplace_back(zkutil::makeCreateRequest(
            fs::path(replica_path) / "parts" / part->name,
            "",
            zkutil::CreateMode::Persistent));
        ops.emplace_back(zkutil::makeCreateRequest(
            fs::path(replica_path) / "parts" / part->name / "columns",
            part->getColumns().toString(),
            zkutil::CreateMode::Persistent));
        ops.emplace_back(zkutil::makeCreateRequest(
            fs::path(replica_path) / "parts" / part->name / "checksums",
            getChecksumsForZooKeeper(part->checksums),
            zkutil::CreateMode::Persistent));
    }
}
ReplicatedMergeTreeAddress StorageReplicatedMergeTree::getReplicatedMergeTreeAddress() const
{
    auto host_port = getContext()->getInterserverIOAddress();
    auto table_id = getStorageID();
    ReplicatedMergeTreeAddress res;
    res.host = host_port.first;
    res.replication_port = host_port.second;
    res.queries_port = getContext()->getTCPPort();
    res.database = table_id.database_name;
    res.table = table_id.table_name;
    res.scheme = getContext()->getInterserverScheme();
    return res;
}
ActionLock StorageReplicatedMergeTree::getActionLock(StorageActionBlockType action_type)