CBQE-927: _async_update with Queue

Change-Id: Ia50549cf82a9090aeb2413fe59ab560f95eeea24
Reviewed-on: http://review.couchbase.org/23953
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>