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. - 

{
            
            
            clearOldPartsFromFilesystem();
            clearOldTemporaryDirectories();
            clearOldWriteAheadLogs();
            clearOldMutations();
            clearEmptyParts();
            return true;
        }, PoolType::MERGE_MUTATE};
    }
    return {};
}
Int64 StorageMergeTree::getCurrentMutationVersion(
    const DataPartPtr & part,
    std::unique_lock<std::mutex> & ) const
{
    auto it = current_mutations_by_version.upper_bound(part->info.getDataVersion());
    if (it == current_mutations_by_version.begin())
        return 0;
    --it;
    return it->first;
}
void StorageMergeTree::clearOldMutations(bool truncate)
{
    const auto settings = getSettings();
    if (!truncate && !settings->finished_mutations_to_keep)
        return;
    std::vector<MergeTreeMutationEntry> mutations_to_delete;
    {