update json_rpc_api.md considering http2

[winteraz]

http2 supports "Allows multiple requests across a single connection" so I believe this should be updated as well as this "Scales well with large numbers of requests"

https://github.com/btcsuite/btcd/blob/master/docs/json_rpc_api.md

[Roasbeef]

There's a contributor working on adding a gRPC server which uses HTTP/2.