Don't pull attachments we already have for a given revpos:

* Don't send atts_since when not pulling attachments inline (it doesn't make sense and will cause
  the attachments to be inlined anyway)

* Don't check "stub" value when not pulling attachments inline (they will always be stubbed, by
  definition)

* Use revpos of rev being added to determine whether we already have the attachments. Do this by
  looking back through the documentRevs object to see if we have the attachment for that generation
  in our bit of the tree.