keepalives

[Robert Olson]

It is likely important that we retain a soft-state flavor to the venues 
registrations in order to flush out dead clients or those on the other side 
of a network partition.

The prototype used a venue service callback, invoked peridiocally from the 
client, to achieve this. If the server noted a client hadn't invoked 
keepalive() lately, it cleared out the client state.

We can either use this mechanism or perhaps something based on the event 
channel for this.

--bob