MB-7509: Skip merging checkpoint end items in chk queues

Change-Id: Idd73a04fc0ff8cce774ee956c006b879bfb1fec5
Reviewed-on: http://review.couchbase.org/24675
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>