Make rendezvous servers more resilient

[robertsdotpm]

The current design works well for multiple servers used to bootstrap a standard flooding-based p2p network (just add servers proportional to network size, have the nodes split across servers and randomly select servers) but for hole punching all nodes need to be on the same server. Think of a way that allows this to scale.

(Won't be used for current client as it will likely be replaced with something else but if its an important issue to note.)