Compress request & response byte formats

We read keys from a msgpack map in order. We can translate this to an array instead where we still read in order, but this will reduce the byte size of our requests and responses.

* Requests will shrink by 2 ("id") + 7 ("payload") + an optional 6 ("header"): 9 to 15 bytes.
* Responses will shrink by 2 ("id") + 9 ("origin_id") + 7 ("payload") + an optional 6 ("header"): 18 to 24 bytes.