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. 

{
    ReadBufferFromFile in(path);
    String contents;
    readStringUntilEOF(contents, in);
    return contents;
}
static int readNumber(const String & path)
{
    ReadBufferFromFile in(path);
    int result;
    readText(result, in);
    return result;
}
#endif
static void sanityChecks(Server & server)