Map<WAL, RollController> getWalNeedsRoll() {
    return this.wals;
  }
Runs periodically to determine if the WAL should be rolled. NOTE: This class extends Thread
rather than Chore because the sleep time can be interrupted when there is something to do, rather
than the Chore sleep time which is invariant. TODO: change to a pool of threads
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/LogRoller.java