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 policy = getStoragePolicy();
    if (ttl.destination_type == DataDestinationType::VOLUME)
    {
        for (const auto & disk : policy->getVolumeByName(ttl.destination_name)->getDisks())
            if (disk->getName() == part.volume->getDisk()->getName())
                return true;
    }
    else if (ttl.destination_type == DataDestinationType::DISK)
        return policy->getDiskByName(ttl.destination_name)->getName() == part.volume->getDisk()->getName();
    return false;
}
CompressionCodecPtr MergeTreeData::getCompressionCodecForPart(size_t part_size_compressed, const IMergeTreeDataPart::TTLInfos & ttl_infos, time_t current_time) const