MB-11661 Remove unreferenced closed checkpoints by item pager.

This change allows the item pager to remove unreferenced closed
checkpoints, so that it can free some memory before attempting to
eject items from the vbucket hash table. In this way, we can
avoid ejecting innocent (frequently accessed) items from hashtable.

Change-Id: I1ec971ce6de348f4320679f18d844c3ac548345d
Reviewed-on: http://review.couchbase.org/39335
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>