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/proxy/grpc_test.go

@@ -0,0 +1,71 @@
+package proxy
+
+import (
+	"testing"
+	"time"
+
+	"github.com/coredns/coredns/plugin/pkg/healthcheck"
+
+	"google.golang.org/grpc/grpclog"
+)
+
+func pool() []*healthcheck.UpstreamHost {
+	return []*healthcheck.UpstreamHost{
+		{
+			Name: "localhost:10053",
+		},
+		{
+			Name: "localhost:10054",
+		},
+	}
+}
+
+func TestStartupShutdown(t *testing.T) {
+	grpclog.SetLogger(discard{})
+
+	upstream := &staticUpstream{
+		from: ".",
+		HealthCheck: healthcheck.HealthCheck{
+			Hosts:       pool(),
+			FailTimeout: 10 * time.Second,
+			Future:      60 * time.Second,
+			MaxFails:    1,
+		},
+	}
+	g := newGrpcClient(nil, upstream)
+	upstream.ex = g
+
+	p := &Proxy{}
+	p.Upstreams = &[]Upstream{upstream}
+
+	err := g.OnStartup(p)
+	if err != nil {
+		t.Errorf("Error starting grpc client exchanger: %s", err)
+		return
+	}
+	if len(g.clients) != len(pool()) {
+		t.Errorf("Expected %d grpc clients but found %d", len(pool()), len(g.clients))
+	}
+
+	err = g.OnShutdown(p)
+	if err != nil {
+		t.Errorf("Error stopping grpc client exchanger: %s", err)
+		return
+	}
+	if len(g.clients) != 0 {
+		t.Errorf("Shutdown didn't remove clients, found %d", len(g.clients))
+	}
+	if len(g.conns) != 0 {
+		t.Errorf("Shutdown didn't remove conns, found %d", len(g.conns))
+	}
+}
+
+// discard is a Logger that outputs nothing.
+type discard struct{}
+
+func (d discard) Fatal(args ...interface{})                 {}
+func (d discard) Fatalf(format string, args ...interface{}) {}
+func (d discard) Fatalln(args ...interface{})               {}
+func (d discard) Print(args ...interface{})                 {}
+func (d discard) Printf(format string, args ...interface{}) {}
+func (d discard) Println(args ...interface{})               {}