Add a cursor index type

Change-Id: I15f0d62228bbcc6d399f1fbd7bd190086f344687
Reviewed-on: http://review.couchbase.org/29571
Tested-by: buildbot <build@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>