cursor prefetch with bounded lag

Nadie ha tomado este issue todavía.

EDIT: nvm, looks like there's really no mechanism in the pgwire protocol that would support this behavior.

Would it at all be possible to support prefetching a batch of up to N rows via a cursor, as compared to waiting for exactly N rows to be available?

In other words, I would like to have a `max_lag` parameter similar to `timeout`, but with different semantics. If after `max_lag` at least 1 row, yet less than `prefetch` rows are available, do return the available rows.

Current api is:

```
cursor(query, *args, prefetch=None, timeout=None, record_class=None)
```

I would like it to be augmented as:

```
cursor(query, *args, prefetch=None, max_lag=None, timeout=None, record_class=None)
```

Is there any intrinsic limitation that would make this not worth the effort or simply there has been no interest for anything like that?

Thank you!

Comienza verificando la limitación de pgwire indicada en el issue y revisando la API de cursor existente descrita en el informe. No se nombran archivos ni tests, y es posible que el comportamiento solicitado no sea implementable; para completarlo sería necesario contar con un diseño confirmado y compatible con el protocolo, así como con un alcance de proyecto correspondiente.

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