Get an expiry buffer.
function expiry(expires) {
  var buffer
    , int64;
  if(expires % 1000 === 0) {
    buffer = new Buffer(5);
    buffer.writeUInt8(0xFD, 0);
    buffer.writeInt32LE(expires / 1000, 1);
  }else{
    buffer = new Buffer(9);
    int64 = new Int64(expires);
    buffer.writeUInt8(0xFC, 0);
    buffer.writeUInt32LE(int64.low32(), 1);
    buffer.writeUInt32LE(int64.high32(), 5);
  }
  return buffer;
}