function GetheadersMessage(arg, options) {
  Message.call(this, options);
  this.command = 'getheaders';
  this.version = options.protocolVersion;
  if (!arg) {
    arg = {};
  }
  arg = utils.sanitizeStartStop(arg);
  this.starts = arg.starts;
  this.stop = arg.stop;
}
Query another peer about block headers. It can query for multiple block hashes,
and the response will contain all the chains of blocks starting from those
hashes.
@param {Object=} options
@param {Array=} options.starts - Array of buffers or strings with the starting block hashes
@param {Buffer=} options.stop - Hash of the last block
@extends Message
@constructor