MB-11799 Let the flusher update the last persisted checkpoint id.

This change makes sure that only the flusher should update the
last persisted checkpoint id. This is required to prevent the
checkpoint remover task from removing checkpoints that are not
persisted yet.

Change-Id: I7577928b6fad7033738ce2ac8092c90e7774b87d
Reviewed-on: http://review.couchbase.org/40059
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>