MB-8930 Stat for max number of bg fetches in the queue seen so far

Change-Id: Ice13a5cccf538b01350ef538d5d09fc6215d1760
Reviewed-on: http://review.couchbase.org/28490
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>