scanner: Support nil for low and high range keys

Change-Id: Ib4441bd60222bf3334e4113a8ab70518b1505af5
Reviewed-on: http://review.couchbase.org/43476
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>