@Override
  public void addEntry(Offset offset, StoreKey key) {
    addEntry(offset, key, null);
  }
A mock journal that makes use of Journal in the background, and provides support for pausing and
resuming the addition of entries to the backing Journal. This can be used to simulate the race
condition of an entry getting added to the index but not yet to the journal.
ambry-store/src/test/java/com/github/ambry/store/MockJournal.java