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/exchanger.go

@@ -0,0 +1,22 @@
+package proxy
+
+import (
+	"context"
+
+	"github.com/coredns/coredns/request"
+	"github.com/miekg/dns"
+)
+
+// Exchanger is an interface that specifies a type implementing a DNS resolver that
+// can use whatever transport it likes.
+type Exchanger interface {
+	Exchange(ctx context.Context, addr string, state request.Request) (*dns.Msg, error)
+	Protocol() string
+
+	// Transport returns the only transport protocol used by this Exchanger or "".
+	// If the return value is "", Exchange must use `state.Proto()`.
+	Transport() string
+
+	OnStartup(*Proxy) error
+	OnShutdown(*Proxy) error
+}