MB-8259 Reflect checkpoint collasping impact on disk queue size.

Multiple checkpoints on a replica vbucket can be collapsed into
a single checkpoint to reduce the memory overhead. We should
reflect the checkpoint collapsing impact on the disk queue size
stat.

Change-Id: I1b4d3fcd0055da160126dacf1a7e12bace2c550c
Reviewed-on: http://review.couchbase.org/26621
Reviewed-by: Jin Lim <jin@couchbase.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>