o Minor features:
    - When we run out of usable circuit IDs on a channel, log only one
      warning for the whole channel, and include a description of
      how many circuits there were on the channel. Fix for part of ticket
      #11553.


  o Major features (performance):
    - Avoid wasting cycles looking for usable circuit IDs. Previously,
      when allocating a new circuit ID, we would in the worst case do a
      linear scan over the entire possible range of circuit IDs before
      deciding that we had exhausted our possibilities. Now, we
      try 64 circuit IDs at random before deciding that we probably
      won't succeed. Fix for a possible root cause of ticket
      #11553.