MB-7685: Refactor BGFetchCallback task

Change-Id: Ide033f5469a5aaa2f9efc870bb290a63299d84ba
Reviewed-on: http://review.couchbase.org/24899
Reviewed-by: Jin Lim <jin@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>