Remove DispatchRpc or make it useful

[maackle]

The DispatchRpc trait is mostly redundant, at one point it was the trait for implementing a custom RPC dispatcher, but now IoHandler handles that completely. So now it's just a useless wrapper around an IoHandler, but that is maybe where we can put the more general handling for requests to get us away from only JSON-RPC, i.e. if we want a simple HttpInterface that doesn't need RPC-JSON

Spun off of #661