Reduce amount of memory that can accumulate in a row batch.

This reduces memory usage noticeably for selective queries and should
have no perf penalty. The rows should have be very big (100kbs) before
this might matter.

Change-Id: I58c3feb4ffc1c4c99724c211428e53fca8bc96d8
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1854
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins