Support re-publication of rendezvous messages

[willscott]

The contents of messages in a topic should be re-publishable, to facilitate longer-latency synchronization.

One of the things preventing this at the moment is that re-publication of the same message will use the same buckets, and repeated writes to a fixed location will look odd.

One way to fix this is to include the 'epoch' in the hashed random seed used to determine bucket location. That would allow the same sequence umber within a topic to be republished in a subsequent epoch, because both the location will still appear unlinked, and a new nonce would mean the contents are a different, indistinguishable random.