`UnionListWriter#startList` points the inner writer at the wrong position

[nbauernfeind]

UnionListWriter's impl:

  @Override
  public void startList() {
    vector.startNewValue(idx());
    writer.setPosition(vector.getOffsetBuffer().getInt((idx() + 1L) * OFFSET_WIDTH));
    listStarted = true;
  }

Whereas UnionLargeListViewWriter:

  @Override
  public void startList() {
    vector.startNewValue(idx());
    writer.setPosition(vector.getOffsetBuffer().getInt((idx()) * OFFSET_WIDTH));
    listStarted = true;
  }

Note that the issue is that UnionListWriter fetches the offset of idx() + 1 -- not the offset of idx() like the Large impl.

Also note that startNewValue returns the position that the writer can be set to; we don't need to poke into the offset buffer.

[nbauernfeind]

Possibly this is the cause of #296.

[nbauernfeind]

Nevermind. While using the returned value from startNewValue is more appropriate, when you start a new value the end offset is initialized to the start offset. While a little confusing, this does not seem to be the issue I'm running into.