PartitionedAgg: consider agg fn pool when choosing partition to spill

When there's memory pressure in PartitionedAggregationNode, we spill
the partition that will release the most memory. However, we weren't
counting the aggregate function's mempool when calculating the largest
partition. This mempool stores intermediate string data, which grows
with the size of the hash table, and is freed when the partition is
spilled.

Change-Id: Ia09beb33e19eecf35446fcf2fa2e4c237a664ec5
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5292
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins