PA: updates to run spilling with minimum required memory.

With this change, we can run partitioned aggs at the minimum required memory. This
value is a constant thats based on the io block size and fanout.

Change-Id: I8555c5216fa14d1ea66dcbcbeff266e2a754a012
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4209
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: Nong Li <nong@cloudera.com>