Remove the word middleware (#1067)

* Rename middleware to plugin

first pass; mostly used 'sed', few spots where I manually changed
text.

This still builds a coredns binary.

* fmt error

* Rename AddMiddleware to AddPlugin

* Readd AddMiddleware to remain backwards compat

plugin/health/healther.go

@@ -0,0 +1,42 @@
+package health
+
+// Healther interface needs to be implemented by each plugin willing to
+// provide healthhceck information to the health plugin. As a second step
+// the plugin needs to registered against the health plugin, by addding
+// it to healthers map. Note this method should return quickly, i.e. just
+// checking a boolean status, as it is called every second from the health
+// plugin.
+type Healther interface {
+	// Health returns a boolean indicating the health status of a plugin.
+	// False indicates unhealthy.
+	Health() bool
+}
+
+// Ok returns the global health status of all plugin configured in this server.
+func (h *health) Ok() bool {
+	h.RLock()
+	defer h.RUnlock()
+	return h.ok
+}
+
+// SetOk sets the global health status of all plugin configured in this server.
+func (h *health) SetOk(ok bool) {
+	h.Lock()
+	defer h.Unlock()
+	h.ok = ok
+}
+
+// poll polls all healthers and sets the global state.
+func (h *health) poll() {
+	for _, m := range h.h {
+		if !m.Health() {
+			h.SetOk(false)
+			return
+		}
+	}
+	h.SetOk(true)
+}
+
+// Middleware that implements the Healther interface.
+// TODO(miek): none yet.
+var healthers = map[string]bool{}