Add kill switch for n_val=1 block get requests

Riak 1.3 will ignore unknown PB params, so it's safe to make the
requests even against 1.3. We still have a runtime kill-switch in case
we need to disable it at runtime. You can revert to the old behavior
with:

`application:set_env(riak_cs, n_val_1_get_requests, false).`