Rendezvous compatibility with Discovery interface

[aschmahmann]

Per conversation with @vyzo we'd like a Discovery client that wraps a rendezvous client.

This means:

• Make a wrapper for Discovery interfaces over the rendezvous client interfaces
• Make a/the Discovery wrapper be a stateful client that receives many peer addresses and returns, through the Discovery interface, a random subset of those peers instead of just returning the peers in the standard order the rendezvous service returns them in.