Pass node in heading callback and fix noinclude section round-tripping

A heading callback was not passing the node reference properly, which is now
fixed. This should enable native separator use preceding headings.

Another fix now ignores mw:Includes/IncludeOnly/End meta tags in the
serializer. These metas don't seem to be needed and have a broken dsr assigned
to them. TODO: Eliminate them?

Overall one additional noinclude-related wt2wt test is greened.

Change-Id: I80a8eb2420aa2da99e643d211d94d3de224ef628