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. - 

{
    return NamesAndTypesList{
        NameAndTypePair("_part", std::make_shared<DataTypeString>()),
        NameAndTypePair("_part_index", std::make_shared<DataTypeUInt64>()),
        NameAndTypePair("_part_uuid", std::make_shared<DataTypeUUID>()),
        NameAndTypePair("_partition_id", std::make_shared<DataTypeString>()),
        NameAndTypePair("_partition_value", getPartitionValueType()),
        NameAndTypePair("_sample_factor", std::make_shared<DataTypeFloat64>()),
    };
}
size_t MergeTreeData::getTotalMergesWithTTLInMergeList() const
{
    return getContext()->getMergeList().getMergesWithTTLCount();
}
void MergeTreeData::addPartContributionToDataVolume(const DataPartPtr & part)
{
    increaseDataVolume(part->getBytesOnDisk(), part->rows_count, 1);
}
void MergeTreeData::removePartContributionToDataVolume(const DataPartPtr & part)
{
    decreaseDataVolume(part->getBytesOnDisk(), part->rows_count, 1);
}
void MergeTreeData::increaseDataVolume(size_t bytes, size_t rows, size_t parts)