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. - 

MergeTreePartition partition(std::move(partition_row));
    String partition_id = partition.getID(*this);
    {
        auto data_parts_lock = lockParts();
        DataPartPtr existing_part_in_partition = getAnyPartInPartition(partition_id, data_parts_lock);
        if (existing_part_in_partition && existing_part_in_partition->partition.value != partition.value)
        {
            WriteBufferFromOwnString buf;
            writeCString("Parsed partition value: ", buf);
            partition.serializeText(*this, buf, format_settings);
            writeCString(" doesn't match partition value for an existing part with the same partition ID: ", buf);
            writeString(existing_part_in_partition->name, buf);
            throw Exception(buf.str(), ErrorCodes::INVALID_PARTITION_VALUE);
        }
    }
    return partition_id;
}
MergeTreeData::DataPartsVector MergeTreeData::getDataPartsVector(
    const DataPartStates & affordable_states, DataPartStateVector * out_states, bool require_projection_parts) const
{
    DataPartsVector res;
    DataPartsVector buf;
    {
        auto lock = lockParts();
        for (auto state : affordable_states)
        {
            auto range = getDataPartsStateRange(state);
            if (require_projection_parts)
            {
                for (const auto & part : range)