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. - 

zkutil::KeeperMultiException::check(code, ops, responses);
    }
    else
    {
        metadata_drop_lock->setAlreadyRemoved();
        completely_removed = true;
        LOG_INFO(logger, "Table {} was successfully removed from ZooKeeper", zookeeper_path);
    }
    return completely_removed;
}
/** Verify that list of columns and table storage_settings_ptr match those specified in ZK (/metadata).
  * If not, throw an exception.
  */
void StorageReplicatedMergeTree::checkTableStructure(const String & zookeeper_prefix, const StorageMetadataPtr & metadata_snapshot)
{
    auto zookeeper = getZooKeeper();
    ReplicatedMergeTreeTableMetadata old_metadata(*this, metadata_snapshot);
    Coordination::Stat metadata_stat;
    String metadata_str = zookeeper->get(fs::path(zookeeper_prefix) / "metadata", &metadata_stat);
    auto metadata_from_zk = ReplicatedMergeTreeTableMetadata::parse(metadata_str);
    old_metadata.checkEquals(metadata_from_zk, metadata_snapshot->getColumns(), getContext());
    Coordination::Stat columns_stat;
    auto columns_from_zk = ColumnsDescription::parse(zookeeper->get(fs::path(zookeeper_prefix) / "columns", &columns_stat));
    const ColumnsDescription & old_columns = metadata_snapshot->getColumns();
    if (columns_from_zk != old_columns)
    {
        throw Exception("Table columns structure in ZooKeeper is different from local table structure", ErrorCodes::INCOMPATIBLE_COLUMNS);
    }
}
void StorageReplicatedMergeTree::setTableStructure(