MB-7656 Add const to nodelist parameter for reducers

Because the nodelist is not supposed to be modified and with
the const qualifier the compiler can detect improper usage of
the node list (reduce function attempting to modify it).
It also serves as developer documentation.

Change-Id: I6ab2fbec9f77a5b1f3d2d850358bfe7c6110ff41
Reviewed-on: http://review.couchbase.org/26668
Reviewed-by: Fulu Li <fulu@couchbase.com>
Reviewed-by: Aaron Miller <apage43@ninjawhale.com>
Tested-by: Aaron Miller <apage43@ninjawhale.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>