MB-8214 set hasWokenTask true when waking up bgfetcher

Change-Id: I7f152bf12edd30bcc92c0c823d7ac3010666268f
Reviewed-on: http://review.couchbase.org/26183
Tested-by: Jin Lim <jin@couchbase.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>