always prioritise acks (and resume)
The former, so that we preserve ack ordering which, while of no
semantic relevance, is good for performance since it allows us to
optimise for the common acks-in-order case.

The latter since it unblocks a queue from delivering messages and thus
is conducive to keeping queue lengths short.

We prioritise acks higher than most other messages since they are the
only high-frequency message which is guaranteed to free up resources.

Prioritising ack and resume all the time is safe since flow control
places an upper bound on how many of these we can get, thus preventing
starvation of senders of lower-priority messages.
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Merge bug25994