New plugin: loop (#1989)

* New plugin: loop

Add a plugin that detects loops. It does this by sending an unique query
to our selves. If we see the query more than twice we stop the process.
If there isn't a loop, the plugin disables it self and becomes a noop
plugin.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/loop/log_test.go

@@ -0,0 +1,5 @@
+package loop
+
+import clog "github.com/coredns/coredns/plugin/pkg/log"
+
+func init() { clog.Discard() }