Test riak binary object reformatting during replication

We currently assume that we can downgrade a cluster's binary object format at any time. This test performs downgrading of each node in the sink cluster while simultaneously writing keys and testing realtime replication.

After all downgrades are complete, it runs fullsync and verifies all keys present.

TODO: test fullsync during downgrading. This will be tricky since downgrading takes nodes offline. I'm not sure what happens to fullsync when nodes go offline.

This test currently fails because the clusters get disconnected when the leader node on cluster B is down'd as part of the reformatting function.