void preLogFlush() {
    /* Save the current start token before the log gets flushed */
    startTokenBeforeLogFlush = startToken;
  }
This method will be called before the log is flushed.
In production, this will be called in the PersistentIndex's IndexPersistor thread.
ambry-store/src/main/java/com/github/ambry/store/HardDeleter.java