CBQE-927: move _async_update in NewUpgradeBaseTest

Change-Id: I0cbc44856211ce9769490673eca827afa4396cd4
Reviewed-on: http://review.couchbase.org/24341
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>