fix kad double read

[turuslan]

• read was called twice, from onMessageRead and onMessageWritten
• simplify and deduplicate read/write request/response
  • server (SessionHost) reads request, writes response, repeats.
  • client (ResponseHandler) writes request, reads response, closes stream.
    • ADD_PROVIDER writes request, (no response), closes stream.
• read uses MessageReadWriterUvarint, but write doesn't use that because Message::serialize adds length prefix.