Lower estimated per-host mem cost for sort nodes

Currently, the per-host mem cost for external sort is the same
as that of top-N, i.e. the size of the entire input.
This patch changes the estimated per-host mem cost to be the amount
of memory required for a 2-phase external sort.

Change-Id: I1bf976fa56a3dddf1a6697c75cf82d3a020cdad2
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3018
Reviewed-by: Marcel Kornacker <marcel@cloudera.com>
Tested-by: jenkins