Add test to check basho/riak_kv#1022 - client/bprop delete mode.

Needs to be separately merged to 2.0.
```verify_delete``` is module local - it's just a little helper.  Your name wins.

I'm using the ```rpc:call````/```riak_client``` dance as the delete modes are deliberately only available from there so they can be used in a future map/reduce job to sweep for tombstones rather than setting very long delete-after times.