Fix race condition in verify_dvv_repl.

Fetch the sink object on each iteration of the wait_until, just in case
that the entire set of siblings didn't make it across the repl link.
This also gives read-repair a chance to happen, in case the version the
sink wrote didn't make it to all replicas.

Seen at http://giddyup.basho.com/#/projects/riak_ee/scorecards/115/115-1729-verify_dvv_repl-centos-6-64/65910

/cc @engelsanchez
Currently, the test is asserting inside the wait function, so it is not incorrect per se. But yes, when it is reworked now, we need to make sure the assertion happens one way or the other.