@Override
  public void delete(K key) {
    if (key == null) {
      throw new NullPointerException("Null key provided");
    }
    map.remove(key);
  }
Implements a {@link KeyValueStore} using an in-memory Java Map.
@param <K>  the type of key
@param <V>  the type of value

TODO: This class is a stop-gap until we implement persistent store creation from TaskContext.
samza-core/src/main/java/org/apache/samza/operators/util/InternalInMemoryStore.java