Reliable ZeroMQ Pub Sub

It's probably a bad idea to use durable sockets for reliable pubsub (I know I documented this in the Guide but that was before it became clear that this functionality is kind of bogus).

Explicit identities are dangerous for servers since they create entropy (state accumulation) unless (and even) if you set strict high-water marks.

They're also a real pain internally and as part of simplifying 0MQ's construction we eventually want to get rid of them.

There are better ways to keep and synchronize state among a set of subscribers. See the detailed Clone pattern in Ch5 of the Guide, for example.

http://zguide.zeromq.org/page:all#A-Shared-Key-Value-Cache-C...