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. - 

{
    DataPartPtr part_to_delete;
    {
        auto lock = lockParts();
        LOG_TRACE(log, "Trying to immediately remove part {}", part->getNameWithState());
        auto it = data_parts_by_info.find(part->info);
        if (it == data_parts_by_info.end() || (*it).get() != part.get())
            throw Exception("Part " + part->name + " doesn't exist", ErrorCodes::LOGICAL_ERROR);
        part.reset();
        if (!((*it)->getState() == DataPartState::Outdated && it->unique()))
            return;
        modifyPartState(it, DataPartState::Deleting);
        part_to_delete = *it;
    }
    try
    {
        part_to_delete->remove();
    }
    catch (...)
    {
        rollbackDeletingParts({part_to_delete});
        throw;
    }
    removePartsFinally({part_to_delete});
    LOG_TRACE(log, "Removed part {}", part_to_delete->name);
}
size_t MergeTreeData::getTotalActiveSizeInBytes() const