MB-11973: Only grab items from the chk manager one chk at a time

We should only grab a single checkpoint at a time from the checkpoint
manager in order to make sure that the replica vbuckets cannot move
into the open checkpoint. This will also help make sure we don't
copy too make items into an upr stream.

Change-Id: I519353df69cf389e6f32f77ffc6afe97dfba7b36
Reviewed-on: http://review.couchbase.org/40765
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>