Add support for multi query views

This adds support for making multiple view queries in one request by
supplying a "queries" list in a POST request body. The primary
implementation was ported over from chttpd_view:multi_query_view/5.

The support added here is two fold. First, we add multi_query_view
support to the single node interface in couch_mrview_http. Second we
add support in couch_mrview_http:view_cb/2 to allow chttpd to utilize
this callback as well.

This also switches updates the #vacc record from setting a multi_query
flag, to instead setting a should_close flag. Now
couch_mrview_http:view_cb/2 will only start a new delayed response if
#vacc.resp is undefined, and similarly, it will only close a delayed
response if #vacc.should_close is true.

COUCHDB-523
COUCHDB-1993