shape of secondary-key for primary index.

secondary-key for primary-index will look-like --- `["<docid>"]`

Change-Id: I3bc94bebbf897bf8b409f1d29d68663c5ae2e7d9
Reviewed-on: http://review.couchbase.org/43473
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>