code: synchronized KeyValue get(long index, String key) throws KayVeeException {
          incrementLastAppliedIndex(index);
  
          String value = entries.get(key);
  
          if (value == null) {
              throw new KeyNotFoundException(key);
          }
  
          return new KeyValue(key, value);
      }
docs: Get the value for a key.
  
  @param index log index >= 0 associated with this command
  @param key non-null (possibly empty) key for which the value should be retrieved
  @return a {@code KeyValue} instance containing the most up-to-date {@code key=>value} pair for {@code key}
  @throws KayVeeException if {@code key} does not exist