WIP: support multi query views

COUCHDB-523
COUCHDB-1993

This is a functional implementation of multi query views on the single
node interface. This is a WIP commit as the implementation is a bit
awkward in some places. In particular, this uses the
couch_mrview_http:view_cb/2 for the entirety of the http response,
which is nice, but it requires introducing state to indicate when
we've reached the last view query, to know when we can complete the
chunked response. Is this better than having multi_query_view/5 finish
the final http chunk and end the response? Pros and cons to either
approach. Once I get some feedback on approach there I'll update
fabric to support this as well.