MB-9938 Fix to the deadlock in doing the bg fetch job.

Change-Id: Ia4cf7a98441812b0e703308bf04125026c4fd7e5
Reviewed-on: http://review.couchbase.org/32561
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>