Add benchmark for scanning MemRowSet
Add benchmark for scanning MemRowSet

When we were working on the demo with Impala, it seemed that scanning
MRS was particularly slow. This adds a benchmark so we can track this
performance and improve it over time.

Change-Id: I22ea873473e73e4fcc67e1c88fe55b39f26d5c29
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3239
Tested-by: jenkins
Reviewed-by: Michael Percy <e07a1fd797b473a9c849e618aa2ce06d8055760b@cloudera.com>

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