Draft LIMIT pushdown to Append and MergeAppend patch

Hi All,

Attached is a draft patch implementing LIMIT pushdown to Append and MergeAppend nodes.

This patch eliminates the need to resort to subqueries to optimise UNIONs.
It also enables more aggressive partition pruning.
Not sure if it causes LIMIT pushdown to foreign partitions though.

Applying this patch causes regressions in:
- postgres_fdw tests
- partitions tests

This is due to subsequent partition pruning applied when LIMIT is pushed down - I guess that’s a (big) win.

I would be happy to hear if the approach is sound.

Thanks,
Michal