Want database query for external IPv6 address allocation

[bnaecker]

We use the NextExternalIp type to generate SQL that creates an external IP address. This takes an IncompleteExternalIp with a bunch of details about the desired address.

Both of these are technically agnostic to the IP version, but as detailed in #1468, this query will fall over when given an IPv6 IP Pool. We need to modify it, specifically the NextExternalIp::push_address_sequence_subquery() method to avoid the use of the generate_series() SQL function. That will fail with IPv6 pools, since the ranges can be huge. We need this to look more like the NextItemSelfJoined query, though we can't use that directly since we don't have the first / last address of all the ranges in the pool in the application code at the time we do the query.

This is highly related to #1468 and #1371.