Add aggregation function SET_UNION

[prithvip]

== RELEASE NOTES ==

General Changes
* Add aggregation function SET_UNION 

[kaikalur]

I wonder if you can just extend SET_AGG to simply override the input method. Everything else should be the same. At the very least, you can use those classes. You don't need the new state and factory classes, I don't think.

[kaikalur]

See my comment about reusing the State and Factory functions from SET_AGG for this as well.

[prithvip]

See my comment about reusing the State and Factory functions from SET_AGG for this as well.

Thanks for the review @kaikalur. Actually, in my initial PR, I was re-using the state and factory functions from set_agg but I changed it because I thought that it might be better to separate it out ,since its rare for the other aggregation functions share state, and these classes are rather small anyways. If you feel strongly about it, I can revert to how it was before and directly use SET_AGG state and factory.

[kaikalur]

See my comment about reusing the State and Factory functions from SET_AGG for this as well.

Thanks for the review @kaikalur. Actually, in my initial PR, I was re-using the state and factory functions from set_agg but I changed it because I thought that it might be better to separate it out ,since its rare for the other aggregation functions share state, and these classes are rather small anyways. If you feel strongly about it, I can revert to how it was before and directly use SET_AGG state and factory.

It's code hygiene. Something doing the exact same thing should not be copied. In fact, I tried very hard to change MAP_AGG to use the set stuffs for keys but it became too complicated so I didn't.

Also note that they are not sharing state - they are just sharing code :)

[prithvip]

No problem @kaikalur, done.

[kaikalur]

LGTM

[rongrong]

Looks good. Some nits, up to you whether you want to change them.

[rongrong]

Maybe change the commit title and release note to Add aggregation function SET_UNION