Redo Packet Parsing

[edunham]

from a conversation in #bravo (https://gist.github.com/edunham/6856310)

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< simpson>| * Packet parsing needs to be redone. Construct is both
too slow and too unmaintainable. The protocol is big enough that walls
of struct.unpack() would be quite reasonable, and we need to figure out
how to subparse all of the domain-specific formats.

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related: #400