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.
Adjust test to ensure that essential concurrency =< n_val
Fix failure on slow replication

This changes the test assertion so that it retries fetching the value
from the second cluster until it is the expected value, at which point
the test will either pass if the sibling count is reasonable or fail if
it is too damn high.
Merge pull request #715 from basho/bugfix/sdc/dvv-race

Fix race condition in verify_dvv_repl.