Simplify the addNewCheckpoint function

Since we keep all cursors in the same list we no longer need to
treat the persistence cursor as special.

Change-Id: Idadb06b873b31587f2eb3d4f57f485165ff1a0e0
Reviewed-on: http://review.couchbase.org/42928
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>