MB-11333: Snapshot marker should account for the extras section

We recently added an extras section to the snapshot marker and need
to account for it when doing flow control.

Change-Id: I17c8122f87d0b63da15e8f5f28c21e9ae494d081
Reviewed-on: http://review.couchbase.org/37932
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>