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. - 

entry_copy.update_time = std::time(nullptr);
                zk->set(entry_copy.znode_path, entry_copy.toString(), entry_copy.version);
            }
        }
        break;
        case EntryState::DESTINATION_ATTACH:
        {
            
            {
                Coordination::Requests ops;
                ops.emplace_back(zkutil::makeCheckRequest(entry.znode_path, entry.version));
                auto part = storage.getActiveContainingPart(entry.part_name);
                
                
                auto block_number_lock = storage.allocateBlockNumber(part->info.partition_id, zk, "", entry.to_shard);
                auto block_number = block_number_lock->getNumber();
                auto part_info = part->info;
                part_info.min_block = block_number;
                part_info.max_block = block_number;
                part_info.level = 0;
                part_info.mutation = 0;
                
                ReplicatedMergeTreeLogEntryData log_entry;
                log_entry.type = ReplicatedMergeTreeLogEntryData::ATTACH_PART;
                log_entry.part_checksum = part->checksums.getTotalChecksumHex();
                log_entry.create_time = std::time(nullptr);
                log_entry.new_part_name = part_info.getPartName();
                ops.emplace_back(zkutil::makeSetRequest(entry.to_shard + "/log", "", -1));
                ops.emplace_back(zkutil::makeCreateRequest(
                    entry.to_shard + "/log/log-", log_entry.toString(), zkutil::CreateMode::PersistentSequential));