[rpc module]: possibility to re-use modules on different servers.

[niklasad1]

The RpcModule doesn't implement Clone and ideally we should some way to construct it once and re-use it to register on several servers such as running the servers on different ports or something similar.

let module = RpcModule::new();
ws_server.register_module(module.clone());
http_server.register_module(module.clone());

Best way is probably to provide some builder that merges the modules when server.start is called.

[maciejhirsz]

This should be easily achievable by making the Methods use Arc<HashMap<_, _>> internally, using Arc::make_mut for adding new methods. Sync methods will need to change from Box<dyn _> to Arc<dyn _> like async methods, which should also unlock a straight forward way to do method aliasing (#378).