This allows us to check all IP addresses (and address families) for a
given DNS hostname while dynamically discovering new IPs and removing
old ones as they're no longer valid.
Also add a testable example that demonstrates how to use it.
Alternative to #11610
Updates tailscale/corp#16367
Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I6d6f39bafc30e6dfcf6708185d09faee2a374599
Andrew Dunham