MB-9939: worker stats fix, debug for MB-9675

Move tasklog and slow log to the taskQueue so that worker
stats are consistent and allows debugging of low disk utilization
seen in MB-9675
Change-Id: I9184a5b87ea91eebc3db0e7c14b7e406245ea468
Reviewed-on: http://review.couchbase.org/32887
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>