Add per client mem tracking to buffered block mgr.

This also means clients of the block mgr need to delete all blocks in close.
This is less important for sorting since it's typically at the end but will
be useful very soon.

Change-Id: Ia4ee188ad845540039ede5fe410a6048abe2bf5a
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3540
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins