Add roundrobin middleware

This middleware allows you to round robin a/aaaa records in a reply and
maybe more in the future (i.e.) sort a packet?

middleware/loadbalance/loadbalance.md

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+# loadbalance
+
+`loadbalance` acts as a round-robin DNS loadbalancer by randomizing A and AAAA records in the
+message. See [Wikipedia](https://en.wikipedia.org/wiki/Round-robin_DNS) about the pros and cons
+on this setup.
+
+## Syntax
+
+~~~
+loadbalance [policy]
+~~~
+
+* policy is how to balance, the default is "round_robin"
+
+## Examples
+
+~~~
+loadbalance round_robin
+~~~