[Docs] Queue federation - pull messages on demand in batches, tracking local queue backlog

federation makes use of priority adjustment, however with large back logs or low credit or slow consumption, low priority federation consumers (which are fast) will get to consume, which can lead to lots of unnecessary message migration.

throttling via a rate limit help mitigate, but reduces throughput when there is no local demand. (https://issues.redhat.com/browse/ENTMQBR-7385)

With this enhancement, a federation consumer will only attempt to pull a batch of messages when there are no local messages (queue size == 0). Priority can still be in play, the consumer will toggle its credit request.

in the case where only producers are upstream, local consumer will get to consume batches of messages.

there is no mass migration and consequently no need to mass migrate back.

the presence of the consumer is demand, the absence of local messages is the trigger to open that demand channel.