indexer: Use default kvstore for storing index metadata

Change-Id: Ic5f06e218f3e2cb4cdf0286017886af21b39e462
Reviewed-on: http://review.couchbase.org/43292
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>