Ordinary database engine is deprecated but there is no simple mean to change database engine to Atomic for existing databases.

Possible options:
- ALTER DATABASE MODIFY ENGINE query.
- Flag that triggers migration at ClickHouse startup.

The latter should be easier to implement. 

const String & table_zookeeper_path, const ReplicatedMergeTreeLogEntryData & entry, Int64 wait_for_inactive_timeout, const String & error_context)
{
    Strings unfinished_replicas = tryWaitForAllReplicasToProcessLogEntry(table_zookeeper_path, entry, wait_for_inactive_timeout);
    if (unfinished_replicas.empty())
        return;
    throw Exception(ErrorCodes::UNFINISHED, "{}Timeout exceeded while waiting for replicas {} to process entry {}. "
                    "Probably some replicas are inactive", error_context, fmt::join(unfinished_replicas, ", "), entry.znode_name);
}
void StorageReplicatedMergeTree::waitForLogEntryToBeProcessedIfNecessary(const ReplicatedMergeTreeLogEntryData & entry, ContextPtr query_context, const String & error_context)