CBQE-927: move _async_update in NewUpgradeBaseTest

Change-Id: Ie7c0d87c6ffe97da1a6ee8590da3fcdbcc3b4ad0
Reviewed-on: http://review.couchbase.org/24319
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>