MB-7656 Track more recent header position in group snapshot

Change-Id: If58ee859342ae7b1e7c06202b04f699008b2a2c1
Reviewed-on: http://review.couchbase.org/28817
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>