'Decode an incoming crawl request message.
:param placeholder:
:param offset: Start of the CrawlRequest message in the data.
:param data: ByteStream containing the message.
:return: (offset, CrawlRequest.impl)'

@staticmethod
def _decode_crawl_request(placeholder, offset, data):

    if (len(data) < (offset + crawl_request_size)):
        raise DropPacket('Unable    to    decode    the    payload')
    (who, seq, limit) = unpack_from(crawl_request_format, data, offset)
    return ((offset + crawl_request_size), placeholder.meta.payload.implement(seq))
