N1QL: Export Indexprovider.

Change-Id: Ib6b605f0dc4779015a54b385b64c17b7753e452b
Reviewed-on: http://review.couchbase.org/44230
Reviewed-by: Manik Taneja <manik@couchbase.com>
Tested-by: Manik Taneja <manik@couchbase.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>