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. - 

Strings parts_to_fetch;
    for (const String & missing_name : expected_parts)
        if (!getActiveContainingPart(missing_name))
            parts_to_fetch.push_back(missing_name);
    /** To check the adequacy, for the parts that are in the FS, but not in ZK, we will only consider not the most recent parts.
      * Because unexpected new parts usually arise only because they did not have time to enroll in ZK with a rough restart of the server.
      * It also occurs from deduplicated parts that did not have time to retire.
      */
    size_t unexpected_parts_nonnew = 0;
    UInt64 unexpected_parts_nonnew_rows = 0;
    UInt64 unexpected_parts_rows = 0;
    for (const auto & part : unexpected_parts)
    {
        if (part->info.level > 0)
        {
            ++unexpected_parts_nonnew;
            unexpected_parts_nonnew_rows += part->rows_count;
        }
        unexpected_parts_rows += part->rows_count;
    }
    
    auto get_blocks_count_in_data_part = [&] (const String & part_name) -> UInt64
    {
        MergeTreePartInfo part_info;
        if (MergeTreePartInfo::tryParsePartName(part_name, &part_info, format_version))
            return part_info.getBlocksCount();
        LOG_ERROR(log, "Unexpected part name: {}", part_name);
        return 0;
    };
    UInt64 parts_to_fetch_blocks = 0;