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
Funny test. Not related to the fix, but technically we don't guarantee no siblings here, right? Even without handoff weirdness, we could even see n_val siblings here and it wouldn't mean DVV is broken. Same with the check below when fetching from ClientB.