Don't require a checkpoint id when closing the open checkpoint

Change-Id: I2780e79351cc4a4f9746f56a6736e45a67ab691e
Reviewed-on: http://review.couchbase.org/42930
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>