Fix CHECK failure in consensus when vlog is enabled
Fix CHECK failure in consensus when vlog is enabled

LogPrefixUnlocked() requires a lock we do not have.

Change-Id: Iaeac795de28008bc27b9440de67d82227e908f7d
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3695
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@cloudera.com>
Tested-by: jenkins

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