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_DEBUG(log, "Cloning {} parts", src_all_parts.size());
    static const String TMP_PREFIX = "tmp_move_from_";
    auto zookeeper = getZooKeeper();
    
    MergeTreePartInfo drop_range;
    std::optional<EphemeralLockInZooKeeper> delimiting_block_lock;
    getFakePartCoveringAllPartsInPartition(partition_id, drop_range, delimiting_block_lock, true);
    String drop_range_fake_part_name = getPartNamePossiblyFake(format_version, drop_range);
    
    String alter_partition_version_path = dest_table_storage->zookeeper_path + "/alter_partition_version";
    Coordination::Stat alter_partition_version_stat;
    zookeeper->get(alter_partition_version_path, &alter_partition_version_stat);
    for (const auto & src_part : src_all_parts)
    {
        if (!dest_table_storage->canReplacePartition(src_part))
            throw Exception(
                "Cannot move partition '" + partition_id + "' because part '" + src_part->name + "' has inconsistent granularity with table",
                ErrorCodes::LOGICAL_ERROR);
        String hash_hex = src_part->checksums.getTotalChecksumHex();
        String block_id_path;
        auto lock = dest_table_storage->allocateBlockNumber(partition_id, zookeeper, block_id_path);
        if (!lock)
        {
            LOG_INFO(log, "Part {} (hash {}) has been already attached", src_part->name, hash_hex);
            continue;
        }
        UInt64 index = lock->getNumber();
        MergeTreePartInfo dst_part_info(partition_id, index, index, src_part->info.level);
        auto dst_part = dest_table_storage->cloneAndLoadDataPartOnSameDisk(src_part, TMP_PREFIX, dst_part_info, dest_metadata_snapshot);
        src_parts.emplace_back(src_part);