Protocol sequence diagrams for ZKAPs

I replaced the very out-of-date "architecture overview" with a couple of protocol sequence diagrams that should more accurately represent how the system currently works (I speculated on some near-future changes).

I produced these to help focus my thinking on https://whetstone.privatestorage.io/privatestorage/privatestoragedesktop/-/issues/178 (but of course the diagrams themselves do not constitute a failsafe payment system).