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. - 

const PartitionCommands & commands, const StorageMetadataPtr & , const Settings & settings) const
{
    for (const auto & command : commands)
    {
        if (command.type == PartitionCommand::DROP_DETACHED_PARTITION
            && !settings.allow_drop_detached)
            throw DB::Exception("Cannot execute query: DROP DETACHED PART is disabled "
                                "(see allow_drop_detached setting)", ErrorCodes::SUPPORT_IS_DISABLED);
        if (command.partition && command.type != PartitionCommand::DROP_DETACHED_PARTITION)
        {
            if (command.part)
            {
                auto part_name = command.partition->as<ASTLiteral &>().value.safeGet<String>();
                
                MergeTreePartInfo::fromPartName(part_name, format_version);
            }
            else
            {
                
                getPartitionIDFromQuery(command.partition, getContext());
            }
        }
    }
}
void MergeTreeData::checkPartitionCanBeDropped(const ASTPtr & partition)