Pipelines

Rework the pub/sub interfaces to allow for the concept of pipelines.

The first example is to create pipelines that compress and then send, ex zlib://|tcp://... and subscriptions that do the reverse tcp://...|zlib://

Rework the subscription plugin interface.

• the higher level pub_sub::Subscriber holds all the subscription objects, not the plugins
• allow for the idea of a source (something that spontaneously creates messages) or a filter (something that receives messages, process them, and send them on).