Fix parsing MAM IQs

prosody includes the supplied queryid in the `<fin>` tag, but this is not XEP compliant.

We set the queryid attr so we can match the MAM Messages to our query.
Thats the only purpose, it is not used to identify the iq result at the end of the query.
For that purpose is the `id` attr on the `<iq>` node.

So dont try to parse `queryid` from the `<fin>` tag.