@Override
      public Void call() throws Exception {
        LOG.info("Execute subprocedure on " + rss.getServerName().toString());
        return null;
      }
Callable task. TODO. We don't need a thread pool to execute roll log. This can be simplified
with no use of subprocedurepool.
hbase-server/src/test/java/org/apache/hadoop/hbase/procedure/SimpleRSProcedureManager.java