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. - 

/
    part_log_elem.duration_ms = elapsed_ns / 1000000;
    part_log_elem.database_name = table_id.database_name;
    part_log_elem.table_name = table_id.table_name;
    part_log_elem.partition_id = MergeTreePartInfo::fromPartName(new_part_name, format_version).partition_id;
    part_log_elem.part_name = new_part_name;
    if (result_part)
    {
        part_log_elem.path_on_disk = result_part->getFullPath();
        part_log_elem.bytes_compressed_on_disk = result_part->getBytesOnDisk();
        part_log_elem.rows = result_part->rows_count;
    }
    part_log_elem.source_part_names.reserve(source_parts.size());
    for (const auto & source_part : source_parts)
        part_log_elem.source_part_names.push_back(source_part->name);
    if (merge_entry)
    {
        part_log_elem.rows_read = (*merge_entry)->rows_read;
        part_log_elem.bytes_read_uncompressed = (*merge_entry)->bytes_read_uncompressed;
        part_log_elem.rows = (*merge_entry)->rows_written;
        part_log_elem.bytes_uncompressed = (*merge_entry)->bytes_written_uncompressed;
        part_log_elem.peak_memory_usage = (*merge_entry)->memory_tracker.getPeak();
    }
    part_log->add(part_log_elem);
}
catch (...)
{
    tryLogCurrentException(log, __PRETTY_FUNCTION__);
}
StorageMergeTree::PinnedPartUUIDsPtr MergeTreeData::getPinnedPartUUIDs() const