CBQE-1099 Fix hang with CreateUpdateDeleteExpireDuringBackup test

better to use already available async method rather than a thread+sync method.
sync method inturns call async method only. Too many threads seem to be causing issue.

Change-Id: I937923cae600af7e55eb0981ad0602cdddd88442
Reviewed-on: http://review.couchbase.org/25048
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>