Use of custom query/placeholder for external dictionaries.
The `placeholder` is similar to `update_field`

Current implementation supports only simple table field like 
`SOURCE(CLICKHOUSE(... update_field 'added_time' update_lag 15))`
but miss support for complex/compound expressions (that for example could use index more efficiently).

Example: 
query = "
    ....
    from t1
    where :placeholder between col1 and col2 and col3=42
    "
invalidate_query=
placeholder_query = "select max(col1) from t1 ...",
placeholder_init_str = "toDateTime('1970-01-01')"

So initial value of placeholder is calculated from `placeholder_init_str`,
next value from `invalidate_query`
and could be used in query.

By introducing this feature work with dictionary sources will be more flexible and general.
writeQuoted(db, out);
            writeChar('.', out);
        }
        if (!schema.empty())
        {
            writeQuoted(schema, out);
            writeChar('.', out);
        }
        writeQuoted(table, out);
        writeString(" WHERE ", out);
        if (!where.empty())
        {
            if (method != CASSANDRA_SEPARATE_PARTITION_KEY)
                writeString("(", out);
            writeString(where, out);
            if (method != CASSANDRA_SEPARATE_PARTITION_KEY)
                writeString(") AND (", out);
            else
                writeString(" AND ", out);
        }
        composeKeysCondition(key_columns, requested_rows, method, partition_key_prefix, out);
        writeString(";", out);
        return out.str();
    }
    else
    {
        writeString(query, out);
        auto condition_position = query.find(CONDITION_PLACEHOLDER_TO_REPLACE_VALUE);
        if (condition_position == std::string::npos)
        {