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. - 

Strings block_id_paths;
    Strings part_checksums;
    std::vector<EphemeralLockInZooKeeper> ephemeral_locks;
    LOG_DEBUG(log, "Cloning {} parts", src_all_parts.size());
    static const String TMP_PREFIX = "tmp_replace_from_";
    auto zookeeper = getZooKeeper();
    String alter_partition_version_path = zookeeper_path + "/alter_partition_version";
    Coordination::Stat alter_partition_version_stat;
    zookeeper->get(alter_partition_version_path, &alter_partition_version_stat);
    
    
    
    
    MergeTreePartInfo drop_range;
    std::optional<EphemeralLockInZooKeeper> delimiting_block_lock;
    bool partition_was_empty = !getFakePartCoveringAllPartsInPartition(partition_id, drop_range, delimiting_block_lock, true);
    if (replace && partition_was_empty)
    {
        
        LOG_INFO(log, "Partition {} was empty, REPLACE PARTITION will work as ATTACH PARTITION FROM", drop_range.partition_id);
        replace = false;
    }
    if (!replace)
    {
        
        drop_range = makeDummyDropRangeForMovePartitionOrAttachPartitionFrom(partition_id);
    }
    assert(replace == !LogEntry::ReplaceRangeEntry::isMovePartitionOrAttachFrom(drop_range));
    String drop_range_fake_part_name = getPartNamePossiblyFake(format_version, drop_range);
    for (const auto & src_part : src_all_parts)