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/response.go

@@ -0,0 +1,21 @@
+package proxy
+
+import (
+	"net"
+
+	"github.com/miekg/dns"
+)
+
+type fakeBootWriter struct {
+	dns.ResponseWriter
+}
+
+func (w *fakeBootWriter) LocalAddr() net.Addr {
+	local := net.ParseIP("127.0.0.1")
+	return &net.UDPAddr{IP: local, Port: 53} // Port is not used here
+}
+
+func (w *fakeBootWriter) RemoteAddr() net.Addr {
+	remote := net.ParseIP("8.8.8.8")
+	return &net.UDPAddr{IP: remote, Port: 53} // Port is not used here
+}