A bit tricky, since we get more than one message at a time. However,
this just means we go over quota for a bit, and will get caught when
those are sent (we do this for a single message already, so it's not
that much worse).
Note: this not only limits sending, but it limits the actuall query
processing, which is nice.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>