Apply INSTRM-907 TCP packet merging to the MPS command headers

In INSTRM-907 I added a buffering mechanism to try hard to merge commands split over multiple TCP packets (the Xilinx TCP stack treats the TCP stream as a stream of packets, not bytes). But I did not apply that to the command header. We are seeing split 8-byte header packets at Subaru, and need to protect against them.

Might be slightly trickier than the INSTRM-907 work but not much.