relay: improve the way we allocate channels
relay: improve the way we allocate channels

Now the server allocates a channel randomly from set of available ids
with the shortest possible length. So concurrency=1 will always yield a
channel-id between 1 and 9 (inclusive). If we have 9 simultaneous
sessions, we'll start allocating channels from 10 to 99. 100
simultaneous connections kicks us into the 100-999 bucket, etc.

warner/magic-wormhole,warner/magic-wormhole,negativo/magic-wormhole,warner/magic-wormhole,barseghyanartur/magic-wormhole,warner/magic-wormhole,david415/magic-wormhole,shaunstanislaus/magic-wormhole,idanyelin/magic-wormhole