When LB is updating NICs to point at it, it'd be good to update them in parallel

[martinsawicki]

right now, the updates are sequential
[ref: LoadBalancerImpl#afterCreating()]

[martinsawicki]

attempted, but the basic issue turns out to be that Azure intentionally prevents the NICs of VMs from the same availability set from being updated in parallel (presumably because of the implications of what an availability set is about), so the current behavior seems to be by design.