Default setting for enable_hashagg_disk

This is just a placeholder thread for an open item that I'm adding to
the Open Items list. We can make a decision later.

Now that we have Disk-based Hash Aggregation, there are a lot more
situations where the planner can choose HashAgg. The
enable_hashagg_disk GUC, if set to true, chooses HashAgg based on
costing. If false, it only generates a HashAgg path if it thinks it
will fit in work_mem, similar to the old behavior (though it wlil now
spill to disk if the planner was wrong about it fitting in work_mem).
The current default is true.

I expect this to be a win in a lot of cases, obviously. But as with any
planner change, it will be wrong sometimes. We may want to be
conservative and set the default to false depending on the experience
during beta. I'm inclined to leave it as true for now though, because
that will give us better information upon which to base any decision.

Regards,
Jeff Davis