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This commit changes the message format for the upstream API, to use a
json-encoded dictionary. While the format is not set in stone at this
point, the goal is to slowly move into a proper protocol, with
well-defined fields to the messages, and proper mechanisms to send
commands and receive notification of their completion.

The only current user of this API is the power management piece, and
this change breaks the GRM code maintained outside of this repo. We will
need to reconcile the two implementation once the message protocol gets
more stable.

Related to #1 and #6.