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. - 

{
    auto lock = (acquired_lock) ? DataPartsLock() : lockParts();
    for (const auto & part : remove)
    {
        if (!data_parts_by_info.count(part->info))
            throw Exception("Part " + part->getNameWithState() + " not found in data_parts", ErrorCodes::LOGICAL_ERROR);
        part->assertState({DataPartState::PreCommitted, DataPartState::Committed, DataPartState::Outdated});
    }
    removePartsFromWorkingSet(remove, clear_without_timeout, lock);
}
MergeTreeData::DataPartsVector MergeTreeData::removePartsInRangeFromWorkingSet(const MergeTreePartInfo & drop_range, bool clear_without_timeout,
                                                                               DataPartsLock & lock)
{
    DataPartsVector parts_to_remove;
    if (drop_range.min_block > drop_range.max_block)
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Invalid drop range: {}", drop_range.getPartName());
    auto partition_range = getDataPartsPartitionRange(drop_range.partition_id);
    for (const DataPartPtr & part : partition_range)
    {
        if (part->info.partition_id != drop_range.partition_id)
            throw Exception("Unexpected partition_id of part " + part->name + ". This is a bug.", ErrorCodes::LOGICAL_ERROR);
        if (part->info.min_block < drop_range.min_block)