[slam] Implement SLAM flow control

On any fast network, or with any driver that may drop packets
(e.g. Infiniband, which has very small RX rings), the traditional
usage of the SLAM protocol will result in enormous numbers of packet
drops and a consequent large number of retransmissions.

By adapting the client behaviour, we can force the server to act more
like a multicast TFTP server, with flow control provided by a single
master client.

This behaviour should interoperate with any traditional SLAM client
(e.g. Etherboot 5.4) on the network.  The SLAM protocol isn't actually
documented anywhere, so it's hard to define either behaviour as
compliant or otherwise.

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