Add ability to disable federation (#1604)

* Allow disabling federation

* Don't start federation queues if disabled

* Fix for Go 1.13

internal/setup/federation.go

@@ -0,0 +1,32 @@
+package setup
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"net/http"
+)
+
+// noOpHTTPTransport is used to disable federation.
+var noOpHTTPTransport = &http.Transport{
+	Dial: func(_, _ string) (net.Conn, error) {
+		return nil, fmt.Errorf("federation prohibited by configuration")
+	},
+	DialContext: func(_ context.Context, _, _ string) (net.Conn, error) {
+		return nil, fmt.Errorf("federation prohibited by configuration")
+	},
+	DialTLS: func(_, _ string) (net.Conn, error) {
+		return nil, fmt.Errorf("federation prohibited by configuration")
+	},
+}
+
+func init() {
+	noOpHTTPTransport.RegisterProtocol("matrix", &noOpHTTPRoundTripper{})
+}
+
+type noOpHTTPRoundTripper struct {
+}
+
+func (y *noOpHTTPRoundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
+	return nil, fmt.Errorf("federation prohibited by configuration")
+}