MB-7656 Add index header structure

Change-Id: If97d5011a3422e328dcac97ffcb31cc85aabe47d
Reviewed-on: http://review.couchbase.org/24461
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Fulu Li <fulu@couchbase.com>
Tested-by: Fulu Li <fulu@couchbase.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>