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 merge_blocker = stopMergesAndWait();
            String partition_id = getPartitionIDFromQuery(partition, local_context);
            parts_to_remove = getDataPartsVectorInPartition(MergeTreeDataPartState::Committed, partition_id);
            
            removePartsFromWorkingSet(parts_to_remove, true);
        }
        if (detach)
        {
            
            
            for (const auto & part : parts_to_remove)
            {
                LOG_INFO(log, "Detaching {}", part->relative_path);
                part->makeCloneInDetached("", metadata_snapshot);
            }
        }
        if (deduplication_log)
        {
            for (const auto & part : parts_to_remove)
                deduplication_log->dropPart(part->info);
        }
        if (detach)
            LOG_INFO(log, "Detached {} parts.", parts_to_remove.size());
        else
            LOG_INFO(log, "Removed {} parts.", parts_to_remove.size());
    }
    clearOldPartsFromFilesystem();
}
PartitionCommandsResultInfo StorageMergeTree::attachPartition(