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. - 

*/
    Int64 right;
    Int64 mutation_version;
    {
        auto zookeeper = getZooKeeper();
        delimiting_block_lock = allocateBlockNumber(partition_id, zookeeper);
        right = delimiting_block_lock->getNumber();
        
        
        
        mutation_version = MergeTreePartInfo::MAX_BLOCK_NUMBER;
    }
    if (for_replace_range)
    {
        
        
        
        
        part_info = MergeTreePartInfo(partition_id, left, right, MergeTreePartInfo::MAX_LEVEL, mutation_version);
        return right != 0;
    }
    
    if (right == 0)
        return false;
    --right;
    
    part_info = MergeTreePartInfo(partition_id, left, right, MergeTreePartInfo::MAX_LEVEL, mutation_version);
    return true;
}
void StorageReplicatedMergeTree::dropPartition(const ASTPtr & partition, bool detach, bool drop_part, ContextPtr query_context, bool throw_if_noop)