Update Riak managers to use the v2 client library

The end result of this branch is going to be:
 - [X] `vumi.persist.riak_manager` using Basho's v2 Riak client
 - [ ] `vumi.persis.txriak_manager` using Basho's v2 Riak client through `deferToThread`
 - [ ] some kind of test script that uses a combination of old and new clients to make sure they can interoperate on the same data

Adding support for new features enabled by the new clients in combination with Riak 1.4 is explicitly out of scope for this branch -- we can do that separately.