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. - 

}
            break;
            case PartitionCommand::FETCH_PARTITION:
                fetchPartition(command.partition, metadata_snapshot, command.from_zookeeper_path, command.part, query_context);
                break;
            case PartitionCommand::FREEZE_PARTITION:
            {
                auto lock = lockForShare(query_context->getCurrentQueryId(), query_context->getSettingsRef().lock_acquire_timeout);
                current_command_results = freezePartition(command.partition, metadata_snapshot, command.with_name, query_context, lock);
            }
            break;
            case PartitionCommand::FREEZE_ALL_PARTITIONS:
            {
                auto lock = lockForShare(query_context->getCurrentQueryId(), query_context->getSettingsRef().lock_acquire_timeout);
                current_command_results = freezeAll(command.with_name, metadata_snapshot, query_context, lock);
            }
            break;
            case PartitionCommand::UNFREEZE_PARTITION:
            {
                auto lock = lockForShare(query_context->getCurrentQueryId(), query_context->getSettingsRef().lock_acquire_timeout);
                current_command_results = unfreezePartition(command.partition, command.with_name, query_context, lock);
            }
            break;
            case PartitionCommand::UNFREEZE_ALL_PARTITIONS:
            {
                auto lock = lockForShare(query_context->getCurrentQueryId(), query_context->getSettingsRef().lock_acquire_timeout);
                current_command_results = unfreezeAll(command.with_name, query_context, lock);
            }
            break;
            default: