Allow multi-packet reads

[rowleya]

An enhancement for SCAMP and SARK would be to allow multi-packet reads, which could speed up the extraction of data. This would work as follows:

1. The host sends a read request where the requested data is more than 256 bytes.
2. Instead of failing, the response is sent as a sequence of packets, each with a different sequential sequence number.
3. If any packets are lost, the host can simply request the data again, using the sequence numbers to work out where the holes in the data are.

This allows the whole protocol to be host-controlled and should be backwards-compatible; if a host requests 256 bytes, it will arrive as a single packet. The host can use a sliding window protocol by simply keeping the last sequential sequence number received and then re-requesting any packets after an appropriate time-out.