MB-10489 Don't use std::map::insert() in updating the cursor pos

Using std::map::insert() to update the cursor position will fail
if the cursor alreay exists in the checkpoint manager. Instead,
we should overwrite it with the new cursor object. This is a
regression from the changes that we made in the master branch.

Change-Id: Icf59763fb7d3fd685768373ea9f0636619ee99cb
Reviewed-on: http://review.couchbase.org/34870
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>