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. - 

}
            for (auto & future : remove_futures)
            {
                auto response = future.get();
                if (response.error == Coordination::Error::ZOK || response.error == Coordination::Error::ZNONODE)
                    continue;
                if (Coordination::isHardwareError(response.error))
                {
                    success = false;
                    continue;
                }
                throw Coordination::Exception(response.error);
            }
        }
        catch (Coordination::Exception & e)
        {
            success = false;
            if (Coordination::isHardwareError(e.code))
                tryLogCurrentException(log, __PRETTY_FUNCTION__);
            else
                throw;
        }
        if (!success && num_tries < max_retries)
            std::this_thread::sleep_for(std::chrono::milliseconds(1000));
    }
    return success;
}
void StorageReplicatedMergeTree::removePartsFromZooKeeper(
    zkutil::ZooKeeperPtr & zookeeper, const Strings & part_names, NameSet * parts_should_be_retried)
{