MB-7656 Allow passing arbitrary data to reducers

While not needed for the database reducer/rereducer functions,
this will be used for view reducer/rereducer functions as a means
to pass data between calls and extra error reporting that is too
view specific.

Change-Id: Ib2b802dc8ec4085b22d72f1d0b5f580f1dfec58f
Reviewed-on: http://review.couchbase.org/26669
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>