HardDeletePersistInfo getHardDeleteRecoveryRange() {
    return hardDeleteRecoveryRange;
  }
Prunes the hardDeleteRecoveryRange so that all the information for keys before startTokenSafeToPersist are
removed. This is safe as the hard deleted writes of those keys are guaranteed to have been flushed in the log.
All the entries from startTokenSafeToPersist to endToken must be maintained in hardDeleteRecoveryRange.
hardDeleteRecoveryRange may still have information of keys that have been persisted in certain cases, but this
does not affect the safety.
Note that we don't need any synchronization for this method as the hardDeleteRecoveryRange and all other
variables except startTokenSafeToPersist are only modified by the hardDeleteThread. Since startTokenSafeToPersist
gets modified by the IndexPersistorThread while this operation is in progress, we save it off first and use the
saved off value subsequently.
ambry-store/src/main/java/com/github/ambry/store/HardDeleter.java