Improve explain plans with respect to aggregations.

This patch addresses the following TODOs:
1. Change the AggregationNode's detail label in explain plans
   because they were not correct and possibly misleading.
   We now only have a FINALIZE as detail label for the last
   aggretation step.
2. Print merge versions of aggregate function call expressions
   as aggfn:merge(orig_agg_fn_child) in the explain plan.
3. Use a CountMerge() function instead of doing a FE rewrite to
   sum() for the merge of count and() count(*).

Change-Id: I44b37703fea695c1c42e516701f5280210847ad1
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4091
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins