Get the buffer for a key.

@param obj The object representing the key and value.
@param vonly A boolean that forces only the value to be encoded.
function key(obj, vonly) {
  var buffer = new Buffer(0)
    , item;
  if(obj.expiry !== undefined && !vonly) {
    buffer = this.expiry(obj.expiry);
  }
  item = this[obj.rtype](obj, vonly);
  return Buffer.concat(
    [
      buffer,
      item
    ],
    buffer.length + item.length
  )
}