MB-8439 add decrDiskQueueSize method instead of undercount assert

DiskQueueSuze is no longer a critical variable for flusher and
checkpoint manager's decision making logics. It is simply a stat
indicating current pending disk write.

At any given point if this stat is undercounted we should just log
a warning and continue instead of assert. Otherwise, the assert
could cause a serious data loss for the crashed node.

Change-Id: I97d7b0ed6e6348578c3f35c373de779a21f469ed
Reviewed-on: http://review.couchbase.org/26897
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>