[master] Changes to verify the get of the benchmark returns a valid result.

Author: shaneh

Summary:
Coordinates load generating threads to get accurate benchmarks.
Confirms results returned are not null, are for the key requested,
and are ofthe correct length.

Test Plan:
Ran the benchmark and confirmed the values were created
correctly.

Reviewers: liyintang, kranganathan, adela

Reviewed By: adela

CC: adela

Differential Revision: https://phabricator.fb.com/D735455

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1465284 13f79535-47bb-0310-9956-ffa450edef68