Replace http-json gateway with grpc-web

[avive]

Motivation

While grpc is great and modern clients such as https://github.com/grpc/grpc-web support server-side streaming, if we plan to support the json-http gateway then it should be complete and support steaming and not just queries. Currently, the json-http code only support queries.

Changes

• Update the json-http code to support server-side streaming methods
• Add integration tests to all json-http server-side streaming methods in go-spacemesh

[avive]

I think we should phase out the json-http api gateway altogether for the next major release of the API (version 2.0 for Spacemesh node 0.3).

Light clients (which don't have a good GRPC client lib) can just use grpc-web and public spacemesh API endpoints can choose to add grpc-web support at runtime via an Envoy Proxy or at compile time by compiling the grpc server in the node to support grpc-web (some grpc server libs provide this functionality today. See: https://github.com/improbable-eng/grpc-web)

So, once grpc-web is supported by the server, http/1.1 clients that can't open a socket (such as browser clients) can easily consume the api. I think that the grpc ecosystem is moving to this direction and away from http-json gateways and we should adopt this strategy.