Protobuf + gRPC

[tiero]

Just asking here to see if there is some sort of conceptual ACK for starting working on this
romanz/electrs#227

It Would be nice if both implementations could expose a gRPC endpoint and the API could be defined by a typed schema.

Why?

1. gRPC gives HTTP/2 for free
2. http2 means binary encoding, more efficient than utf8 JSON strings
3. Protobuf is a perfect match, could give backward and forward compatibility when making changes to the APIs
4. Server-side streaming
5. Automatically generate stubs and clients from the single .proto model spec

[shesek]

A gRPC/protobuf interface is something that I'd be willing to accept. It should be put behind a compile-time flag, like the current http/electrum/webhooks flags.

Note that bwt already supports HTTP/2 (via warp/hyper) and already does server-side streaming using Server-Sent Events. The main advantages for me would be protobut instead of json and a more well-defined explicit interface.

I probably won't be prioritizing this myself in the near future, but will gladly review a PR.