Add new algorithms to the NameserverPool for rotating nameservers used when making queries

[bluejekyll]

Is your feature request related to a problem? Please describe.

Some namesevers will rate-limit clients. We should come up with new algorithms and policies for how to choose the a nameserver in the pool for each next query.

Describe the solution you'd like

We should use this thread to create requirements. Today's algorithm is pretty basic. It works by reordering the Nameservers on each request, but does not currently offer any form of rate-limiting or forced rotation. Additionally it would be nice to track the latency to each nameserver and use that for prioritizing requests.

[djc]

#1920 and #1784 made some progress on this, I think.