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. - 

{
    return total_active_size_rows.load(std::memory_order_acquire);
}
size_t MergeTreeData::getPartsCount() const
{
    return total_active_size_parts.load(std::memory_order_acquire);
}
size_t MergeTreeData::getMaxPartsCountForPartitionWithState(DataPartState state) const
{
    auto lock = lockParts();
    size_t res = 0;
    size_t cur_count = 0;
    const String * cur_partition_id = nullptr;
    for (const auto & part : getDataPartsStateRange(state))
    {
        if (cur_partition_id && part->info.partition_id == *cur_partition_id)
        {
            ++cur_count;
        }
        else
        {
            cur_partition_id = &part->info.partition_id;
            cur_count = 1;
        }
        res = std::max(res, cur_count);
    }
    return res;
}
size_t MergeTreeData::getMaxPartsCountForPartition() const