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. - 

{
    if (path.empty())
        throw Exception("ZooKeeper path should not be empty", ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT);
    auto pos = path.find(':');
    if (pos != String::npos)
    {
        return normalizeZooKeeperPath(path.substr(pos + 1, String::npos));
    }
    return normalizeZooKeeperPath(path);
}
static MergeTreePartInfo makeDummyDropRangeForMovePartitionOrAttachPartitionFrom(const String & partition_id)
{
    
    
    
    
    
    
    
    
    MergeTreePartInfo drop_range;
    drop_range.partition_id = partition_id;
    drop_range.min_block = 0;
    drop_range.max_block = 0;
    drop_range.level = 0;
    drop_range.mutation = 0;
    return drop_range;
}
StorageReplicatedMergeTree::StorageReplicatedMergeTree(