issue 441 - upside_down termFieldReader doesn't call Next() early
issue 441 - upside_down termFieldReader doesn't call Next() early

This change to upside_down term-field-reader no longer moves the
underlying iterator forward preemptively.  Instead, it will invoke
Next() on the underlying iterator only when the caller invokes the
term-field-reader's Next().

There's a special case to handle the situation on the first Next()
invocation after the term-field-reader is created.

steveyen/bleve,steveyen/bleve,blevesearch/bleve,slavikm/bleve,blevesearch/bleve,slavikm/bleve