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. - 

throw Exception("Checksums of " + part_desc->src_table_part->name + " is suddenly changed", ErrorCodes::UNFINISHED);
            part_desc->res_part = cloneAndLoadDataPartOnSameDisk(
                part_desc->src_table_part, TMP_PREFIX + "clone_", part_desc->new_part_info, metadata_snapshot);
        }
        else if (!part_desc->replica.empty())
        {
            String source_replica_path = fs::path(zookeeper_path) / "replicas" / part_desc->replica;
            ReplicatedMergeTreeAddress address(getZooKeeper()->get(fs::path(source_replica_path) / "host"));
            auto timeouts = getFetchPartHTTPTimeouts(getContext());
            auto credentials = getContext()->getInterserverCredentials();
            String interserver_scheme = getContext()->getInterserverScheme();
            if (interserver_scheme != address.scheme)
                throw Exception("Interserver schemas are different '" + interserver_scheme + "' != '" + address.scheme + "', can't fetch part from " + address.host, ErrorCodes::LOGICAL_ERROR);
            part_desc->res_part = fetcher.fetchPart(
                metadata_snapshot, getContext(), part_desc->found_new_part_name, source_replica_path,
                address.host, address.replication_port, timeouts, credentials->getUser(), credentials->getPassword(), interserver_scheme, false, TMP_PREFIX + "fetch_");
            
            ProfileEvents::increment(ProfileEvents::ReplicatedPartFetches);
        }
        else
            throw Exception("There is no receipt to produce part " + part_desc->new_part_name + ". This is bug", ErrorCodes::LOGICAL_ERROR);
    };
    
    
    for (PartDescriptionPtr & part_desc : final_parts)
        obtain_part(part_desc);
    MutableDataPartsVector res_parts;
    for (PartDescriptionPtr & part_desc : final_parts)
        res_parts.emplace_back(part_desc->res_part);
    try