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/file/tree/all.go

@@ -0,0 +1,48 @@
+package tree
+
+// All traverses tree and returns all elements
+func (t *Tree) All() []*Elem {
+	if t.Root == nil {
+		return nil
+	}
+	found := t.Root.all(nil)
+	return found
+}
+
+func (n *Node) all(found []*Elem) []*Elem {
+	if n.Left != nil {
+		found = n.Left.all(found)
+	}
+	found = append(found, n.Elem)
+	if n.Right != nil {
+		found = n.Right.all(found)
+	}
+	return found
+}
+
+// Do performs fn on all values stored in the tree. A boolean is returned indicating whether the
+// Do traversal was interrupted by an Operation returning true. If fn alters stored values' sort
+// relationships, future tree operation behaviors are undefined.
+func (t *Tree) Do(fn func(e *Elem) bool) bool {
+	if t.Root == nil {
+		return false
+	}
+	return t.Root.do(fn)
+}
+
+func (n *Node) do(fn func(e *Elem) bool) (done bool) {
+	if n.Left != nil {
+		done = n.Left.do(fn)
+		if done {
+			return
+		}
+	}
+	done = fn(n.Elem)
+	if done {
+		return
+	}
+	if n.Right != nil {
+		done = n.Right.do(fn)
+	}
+	return
+}