scanner: Adapt to the special handling of primary index

Primary index will have keys in the form [pk] instead of
[sk, pk] for secondary indexes.

Change-Id: Iddbb1984391c259e5eb375b0010481ba50fec8c4
Reviewed-on: http://review.couchbase.org/43475
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>