Deduplicate aggregates and transition functions in planner

Hi,

Currently, ExecInitAgg() performs quite a lot of work, to deduplicate 
identical Aggrefs, as well as Aggrefs that can share the same transition 
state. That doesn't really belong in the executor, we should perform 
that work in the planner. It doesn't change from one invocation of the 
plan to another, and it would be nice to reflect the state-sharing in 
the plan costs.

Attached is a patch to do that. It adds two new fields to Aggref, 
'aggno' and 'aggtransno', to identify the unique aggregate and 
transition states. The duplicates are detected, and those filled in, 
early in the planning. Aside from those fields, the planner doesn't pass 
any other new information to to the executor, so the the executor still 
has to do syscache lookups to get the transition, combine etc. functions.

I tried a bigger refactoring at first, to pass more information from the 
planner to the executor, but the patch grew really large before I got 
very far with it. So as the first step, I think we should apply the 
attached patch, and further refactoring can be done after that, if it 
seems worthwhile.

There is one known regression failure, in the 'partition_aggregate' 
test, which is caused by a plan change in one of the queries. The query 
contains a few aggregates, and the planner now detects that some of them 
are identical, which changed the cost estimates, making a different plan 
look cheaper. That's easy to fix, although I'm not sure yet if we should 
accept the new plan, or change the query to still get the old plan.

- Heikki