SeriesBlock and Series "metadata" is confusing

The Series and SeriesBlock objects have "metadata" attributes, but they are usually empty (at least when returned from stream or fetch calls.

It seems like for Series objects the "metadata" is supposed to just be a Channel object for the Series. The attribute should probably just be "channel", and it should not appear as empty when inspecting the object.

It's unclear why the SeriesBlock has a separate metadata attribute, since it appears that it should be the same info in the enclosed Series.metadata attribute, so I propose we just drop this attribute.