Deduplicate RPC types

[roman-khimov]

We have some number of types defined in the rpc package that represent the structures being passed from server to client, but those are mostly used by the client code. At the same time we have similar types defined in the rpc/wrappers that are used by the server code. There is an obvious duplication there but to make things even more fun we have a third definition of these types for test code in the rpc/server_helper_test.go.

I think we should move all of them into one package separate from the rpc logic, that package can be shared between server and client code (that technically are also good candidates for package split).

[roman-khimov]

Take a look at dedup-rpc-types branch. I think we can also separate client and server code into different packages while here.