MB-7729: Fix 100% CPU Issue

The background fetcher jobs todo stat was not being
decremented properly.

Change-Id: I26a35e76074644b63117d348c05123c09691cc44
Reviewed-on: http://review.couchbase.org/26167
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Jin Lim <jin@couchbase.com>