Change units and naming of some counters used in sort

Also differentiates between memory limit and memory used.
Change-Id: Ic5534345830b1c3b5109697a93868eb5d40befda
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3217
Reviewed-by: Srinath Shankar <sshankar@cloudera.com>
Tested-by: jenkins