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

Change-Id: I54c05804957fee635e294d1ae17d0e14369ad808
Reviewed-on: http://review.couchbase.org/26832
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>