MB-7629: Change reduce function signature for view reducers

Change-Id: Id928eb1971401d8d2834be2a95ee8b8f26665216
Reviewed-on: http://review.couchbase.org/26481
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>