Consider phasing-out http-json gateway

[avive]

We should at some point replace the http-json gateway to encourage more clients to use grpc instead of http/json and so a node doesn't need to have the node listen on 2 different ports - one for grpc and one for http-json - reduced surface is always nice.

We should consider adding grpc-web protocol support to the api - this will enable web clients (that can't create an http/2 socket) to communicate with an api endpoint directly and is becoming the standard. There are already easy ways to enable this in Rust (e.g. tonic-web) but I'm not 100% sure about go.