[rabbitmq-discuss] Set 21673 memory limit

[tsuraan]

> The above parameter sets an overall memory limit. The memory available
> to queues is dynamically adjusted to keep within that overall limit.
>
> Early versions of the persister had different controls, so perhaps
> that's what you remember.

Ok, so the cache is now governed by the overall memory limit.  Under
what circumstances will channel.flow be issued now?