MB-7629: Add built-in _sum reduce/rereduce function for views

Change-Id: I05f8bdae91f11f85de802e609c9921062c3a9a47
Reviewed-on: http://review.couchbase.org/26771
Tested-by: Fulu Li <fulu@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>