[ISC-support #17393] Feature request - Global v6 PD pools

Currently it is possible to configure overlapping PD pools between v6 subnets (or even use the same exact PD pool entirely for all subnets).

This 'appears' to work OK. Our review of the situation however was that without more extensive testing and research, we couldn't say 'this is supported' because what we have here is something that is essentially a happy accident. It looks like it does probably work - but only because the check for whether a PD is already allocated, doesn't check the subnet ID!

The statistics on the other hand, are going to be very peculiar, because they'll have allocations for the same space coming from different subnets. They might work if the administrator does a manual combination of them across the subnets, but taken standalone, they won't make much sense.

This is a request for this feature to be tested, stats fixed, and to be properly supported. There is a customer/operational use-case for it explained in Support Ticket #17393