RPC improvements

[Hyodar]

This should be a big change. We're currently using the net/rpc package which offers a pretty basic RPC interface, but it's pretty limited in some scenarios. For example, we can't properly access the sender address without some workarounds and it's not as standard as some other resources. We also want to have more traffic management for DOS protection.
I propose we research our options a bit here. An RPC package that is compatible with net/rpc but adds extra features would be preferrable, but we could also move to gRPC.