schema update: manage internal/external DNS

[davepacheco]

Not exactly part of schema update, but related to the same handoff.

• We should not include new-version Nexus instances in internal or external DNS before the handoff
• We should not include old-version Nexus instances in internal or external DNS after the handoff

[karencfv]

Might need #8863 for this

[karencfv]

Will unassign myself from this for now

[smklein]

Posting this timeline here, as we discussed in chat, along with this excerpt from RFD 588 on the topic:

When updating the contents of external DNS, Nexus will only include the set of Nexus instances currently in charge (namely, having the same nexus_generation as the Nexus that’s doing the execution). This means only the old instances will be in external DNS prior to the handoff and new Nexus instances will be in external DNS after the handoff.

This means:

• We should avoid adding new Nexuses records to DNS until they're ready to be used (post-handoff)
• Old nexuses will remain in DNS during quiesce, and will be removed by the new Nexus instance once it's ready