scanner: Fix scan inclusion to handle compound key indexes

When a lowkey or highkey specified in the scan request has multiple,
fields, that many number of fields should be compared with keys from
the index storage to decide equality.

Change-Id: Ife29c6deb79a9ff325f15dda4fe87cb944ce3e3b
Reviewed-on: http://review.couchbase.org/43843
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>