healhcheck: various cleanups (#1106)

* healhcheck: various cleanups

Network wasn't used. IgnorePaths wasn't used. Move checkdown function to
common function shared between proxy protocols. And some naming fixed.

Also reset the Fails on a succesful healthcheck back to 0.

remove newlines from log

* compile

* fix test

plugin/proxy/down.go

@@ -0,0 +1,30 @@
+package proxy
+
+import (
+	"sync/atomic"
+	"time"
+
+	"github.com/coredns/coredns/plugin/pkg/healthcheck"
+)
+
+// Default CheckDown functions for use in the proxy plugin.
+var checkDownFunc = func(upstream *staticUpstream) healthcheck.UpstreamHostDownFunc {
+	return func(uh *healthcheck.UpstreamHost) bool {
+
+		down := false
+
+		uh.Lock()
+		until := uh.OkUntil
+		uh.Unlock()
+
+		if !until.IsZero() && time.Now().After(until) {
+			down = true
+		}
+
+		fails := atomic.LoadInt32(&uh.Fails)
+		if fails >= upstream.MaxFails && upstream.MaxFails != 0 {
+			down = true
+		}
+		return down
+	}
+}