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. - 

const ASTPtr & new_settings,
        TableLockHolder & )
{
    if (new_settings)
    {
        bool has_storage_policy_changed = false;
        const auto & new_changes = new_settings->as<const ASTSetQuery &>().changes;
        for (const auto & change : new_changes)
        {
            if (change.name == "storage_policy")
            {
                StoragePolicyPtr new_storage_policy = getContext()->getStoragePolicy(change.value.safeGet<String>());
                StoragePolicyPtr old_storage_policy = getStoragePolicy();
                
                if (new_storage_policy != old_storage_policy)
                {
                    checkStoragePolicy(new_storage_policy);
                    std::unordered_set<String> all_diff_disk_names;
                    for (const auto & disk : new_storage_policy->getDisks())
                        all_diff_disk_names.insert(disk->getName());
                    for (const auto & disk : old_storage_policy->getDisks())