Convert consensus-test-util executor back to ThreadPool
Convert consensus-test-util executor back to ThreadPool

One of the last vestiges of the TaskExecutor we are trying to remove.

Change-Id: I67a5615409f1f6baa42feeeea1418a3e4d0adb87
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4910
Tested-by: Michael Percy <e07a1fd797b473a9c849e618aa2ce06d8055760b@cloudera.com>
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@cloudera.com>

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