MB-7685: Refactor the VKeyStatBGFetchCallback task

Change-Id: I9f1db236cd0f2423a27c8e3acaa57c6242ef2b41
Reviewed-on: http://review.couchbase.org/24897
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>