proper round-robining even when blocking

This involved some substantial changes to the queue internal data
structures - mostly by choice; the new design is cleaner:

- We no longer keep a list of consumers in the channel
  records. Now the channel records just contain a consumer count
  instead, and   that's only there for efficiency so we can more
  easily tell when we need to register/unregister with the limiter.

- We now keep *two* consumer queues - one of active consumers
  (that's the one we've always had) and one of blocked consumers.

We round-robin on the first one as before, and move things between the
two queues when blocking/unblocking channels. When doing so the
relative order of a channel's consumers is preserved, so the effects
of any round-robining the active consumers get carried
through to the blocked consumers when they get blocked and then back
to the active consumers when they get unblocked.

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