9nbd handle failover to multiple servers

[garlick]

I thought this code was already in 9nbd, but it looks like it may not have been completed. We need to be able to attach to a comma-separated list of servers so that when one fails, another can take its place provided the identical image is being served there.

What we have now is the ability to start a new session (thread) that reconnects to the same server when the active one times out a request. Should be simple to have the new session pick from a list of available servers.