Support aggregate functions with different intermediate and output types.

As a proof-of-concept, this patch implements avg() with a STRING intermediate
type, and changes variance() to output a DOUBLE.

I tested this change on single-node and distributed plans, with the
partitioned as well as the old aggregation node.

This patch leaves several things for follow-on changes:
- plumb through CHAR as an intermediate type
- modify other builtin aggregtes to use appropriate output/intermediate types
- allow analytic functions to have different output/intermediate types

Change-Id: I8d3396201cb370f44660ab4f7fe10216129abd09
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4016
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins