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/pkg/cache/cache_test.go

@@ -0,0 +1,31 @@
+package cache
+
+import "testing"
+
+func TestCacheAddAndGet(t *testing.T) {
+	c := New(4)
+	c.Add(1, 1)
+
+	if _, found := c.Get(1); !found {
+		t.Fatal("Failed to find inserted record")
+	}
+}
+
+func TestCacheLen(t *testing.T) {
+	c := New(4)
+
+	c.Add(1, 1)
+	if l := c.Len(); l != 1 {
+		t.Fatalf("Cache size should %d, got %d", 1, l)
+	}
+
+	c.Add(1, 1)
+	if l := c.Len(); l != 1 {
+		t.Fatalf("Cache size should %d, got %d", 1, l)
+	}
+
+	c.Add(2, 2)
+	if l := c.Len(); l != 2 {
+		t.Fatalf("Cache size should %d, got %d", 2, l)
+	}
+}