cleanup: doh.powerdns.org is not working anymore (#924)

While there, `.../internal/sessionresolver` => `.../sessionresolver`

See https://github.com/ooni/probe/issues/2255

internal/engine/sessionresolver/jsoncodec.go

@@ -0,0 +1,37 @@
+package sessionresolver
+
+//
+// JSON codec
+//
+
+import "encoding/json"
+
+// jsonCodec encodes to/decodes from JSON.
+type jsonCodec interface {
+	// Encode encodes v as a JSON stream of bytes.
+	Encode(v interface{}) ([]byte, error)
+
+	// Decode decodes b from a JSON stream of bytes.
+	Decode(b []byte, v interface{}) error
+}
+
+// codec always returns a valid jsonCodec.
+func (r *Resolver) codec() jsonCodec {
+	if r.jsonCodec != nil {
+		return r.jsonCodec
+	}
+	return &jsonCodecStdlib{}
+}
+
+// jsonCodecStdlib is the default codec.
+type jsonCodecStdlib struct{}
+
+// Decode implements jsonCodec.Decode.
+func (*jsonCodecStdlib) Decode(b []byte, v interface{}) error {
+	return json.Unmarshal(b, v)
+}
+
+// Encode implements jsonCodec.Encode.
+func (*jsonCodecStdlib) Encode(v interface{}) ([]byte, error) {
+	return json.Marshal(v)
+}