Change headers format to str:json

[fantix]

Headers now has the following format:

struct Header {
  // Header key.
  bytes           key;
  // Header data.
  bytes           value;
}

struct SomeMessage {
  ...
  // A set of message headers.
  uint16          num_headers;
  Header          headers[num_headers];
}

• Header in ErrorMessage is not changed
• ClientHandshake always send b'\0\0' (empty ext headers)
• ServerHandshake no longer has ext headers