KUDU-1491. Address TSAN warning for compaction
KUDU-1491. Address TSAN warning for compaction

This adds a missing memory barrier that was exposed when I removed
TSAN suppressions for mutation lists. Without the patch, TSAN
failed on mt-tablet-test a few percent of the time. With the patch,
I looped it 50 times with no failures:

http://dist-test.cloudera.org/job?job_id=todd.1466638821.4940

Change-Id: I42d2566eca096da172440ce3a88d0393dda9d324
Reviewed-on: http://gerrit.cloudera.org:8080/3454
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>
Tested-by: Kudu Jenkins

EvilMcJerkface/kudu,andrwng/kudu,helifu/kudu,helifu/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,andrwng/kudu,andrwng/kudu,andrwng/kudu,cloudera/kudu,cloudera/kudu,helifu/kudu,InspurUSA/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,andrwng/kudu,InspurUSA/kudu,cloudera/kudu,EvilMcJerkface/kudu,helifu/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,helifu/kudu,EvilMcJerkface/kudu,helifu/kudu,InspurUSA/kudu,cloudera/kudu,cloudera/kudu,helifu/kudu,andrwng/kudu,cloudera/kudu,cloudera/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,andrwng/kudu,InspurUSA/kudu,InspurUSA/kudu,andrwng/kudu,cloudera/kudu,andrwng/kudu,helifu/kudu,cloudera/kudu,helifu/kudu