kh: parse pubkeys at json parsing site, not later
kh: parse pubkeys at json parsing site, not later

This ensures we crash early if the received data is in an unexpected
format or otherwise not sane.

ngzax/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,urbit/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit