Update chttpd_view to use couch_mrview for multi query views

This switches to using the couch_mrview implementation for handling
multi query view requests, both for fetching the views through fabric
and also the http callbacks.

One concern I have with this implementation that needs to be tested,
is whether calling couch_mrview_util:get_view in multi_query_view is
appropriate. What happens when the database or the ddoc does not exist
on the node handling this request? I think the ddoc should be fine as
we load that from the ddoc_cache, but I'm less sure about the db. We
need to make this request so we can perform the validations on the
view, both for the url query params, and also for every set of
additional params provided in the list of queries.

COUCHDB-523
COUCHDB-1993