Consider low startup cost in add_partial_path

Over in the incremental sort patch discussion we found [1] a case
where a higher cost plan ends up being chosen because a low startup
cost partial path is ignored in favor of a lower total cost partial
path and a limit is a applied on top of that which would normal favor
the lower startup cost plan.

45be99f8cd5d606086e0a458c9c72910ba8a613d originally added
`add_partial_path` with the comment:

> Neither do we need to consider startup costs:
> parallelism is only used for plans that will be run to completion.
> Therefore, this routine is much simpler than add_path: it needs to
> consider only pathkeys and total cost.

I'm not entirely sure if that is still true or not--I can't easily
come up with a scenario in which it's not, but I also can't come up
with an inherent reason why such a scenario cannot exist.

We could just continue to include this change as part of the
incremental sort patch itself, but it seemed worth it to me to break
it out for some more targeted discussion, and also include Robert as
the initial author of add_partial_path in the hopes that maybe we
could retrieve some almost 4-year-old memories on why this was
inherently true then, and maybe that would shed some light on whether
it's still inherently true.

I've attached a patch (by Tomas Vondra, also cc'd) to consider startup
cost in add_partial_path, but should we apply the patch we'll also
likely need to apply the same kind of change to
add_partial_path_precheck.

James Coleman

[1]: https://www.postgresql.org/message-id/20190720132244.3vgg2uynfpxh3me5%40development