Add option to keep contact points unresolved and always re-resolve when there's total connectivity loss

I read some more documentation and it looks like this is completely dependent on the underlying OS since the DNS class just invokes the OS DNS subsystem. Maybe it's fine to keep using Dns.GetHostEntry (also looks like the ServicePointManager is only used for HTTP requests).

See https://stackoverflow.com/questions/17454925/does-net-respect-dns-ttl-values
Also see https://stackoverflow.com/questions/6928378/how-to-flush-cache-for-socket-gethostbyname-response for a similar problem in Python

The thing that worries me with that class is that it applies to the entire application so I'll try changing that setting just before the query and resetting it to the original value afterwards I guess.

Hmm interesting, I'll add that to the description and do some tests with it when I pick this up. That isn't available in net standard 1.5 but we are going to drop support for it anyway.

I found this: https://docs.microsoft.com/en-us/dotnet/api/system.net.servicepointmanager.dnsrefreshtimeout?view=netcore-2.2

Unfortunately I can't find any information realated to caching on the microsoft documentation pages...