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. - 

continue;
            if (!is_appropriate_state((*it)->getState()))
            {
                update_error(it);
                continue;
            }
            if ((*it)->info.min_block > pos)
                update_error(it);
            if ((*it)->getState() != DataPartState::Committed)
            {
                addPartContributionToColumnSizes(*it);
                addPartContributionToDataVolume(*it);
                modifyPartState(it, DataPartState::Committed);
            }
            pos = (*it)->info.max_block + 1;
            restored.push_back((*it)->name);
        }
        if (pos != part->info.max_block + 1)
            error = true;
        for (const String & name : restored)
        {
            LOG_INFO(log, "Activated part {}", name);
        }
        if (error)
        {
            LOG_ERROR(log, "The set of parts restored in place of {} looks incomplete. There might or might not be a data loss.{}", part->name, (error_parts.empty() ? "" : " Suspicious parts: " + error_parts));
        }
    }
}
void MergeTreeData::tryRemovePartImmediately(DataPartPtr && part)