code: synchronized KeyValue set(long index, String key, String value) {
          incrementLastAppliedIndex(index);
  
          entries.put(key, value);
  
          return new KeyValue(key, entries.get(key));
      }
docs: Set a key to a value.
  
  @param index log index >= 0 associated with this command
  @param key non-null (possibly empty) key for which the value should be set
  @param value non-null, non-empty value for this key
  @return a {@code KeyValue} instance containing the most up-to-date {@code key=>value} pair for {@code key}