coredns/plugin: Improve weighted endpoint algorithm

**What**

Improve the weighted algorithm used when selecting the most appropriate endpoint to select for weighted record sets.

Currently is uses the same logic as the CoreDNS [loadbalanced plugin](https://github.com/coredns/coredns/tree/master/plugin/loadbalance), but this appears to be quite random in the way it works and there may be a better way to do this to make it more deterministic.

Start with the current weighted endpoint selection implementation and compare its behavior with the linked CoreDNS loadbalance plugin. Define a more deterministic rule for weighted record sets and verify it with representative selection tests; done means the algorithm is no longer effectively random.

Written by the indexing model from the issue text.