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. - 

FutureMergedMutatedPart & future_part_,
    size_t total_size,
    StorageMergeTree & storage_,
    const StorageMetadataPtr & metadata_snapshot,
    bool is_mutation)
    : future_part(future_part_), storage(storage_)
{
    
    
    if (is_mutation)
        reserved_space = storage.tryReserveSpace(total_size, future_part_.parts[0]->volume);
    else
    {
        IMergeTreeDataPart::TTLInfos ttl_infos;
        size_t max_volume_index = 0;
        for (auto & part_ptr : future_part_.parts)
        {
            ttl_infos.update(part_ptr->ttl_infos);
            max_volume_index = std::max(max_volume_index, storage.getStoragePolicy()->getVolumeIndexByDisk(part_ptr->volume->getDisk()));
        }
        reserved_space = storage.balancedReservation(
            metadata_snapshot,
            total_size,
            max_volume_index,
            future_part.name,
            future_part.part_info,
            future_part.parts,
            &tagger,
            &ttl_infos);
        if (!reserved_space)