Remove the JsonRpcResponses type

[tomaka]

I'm not very happy with the JsonRpcResponses type in the public API of the light client.

I think that it should instead be a pub fn next_json_rpc_response(&self, ChainId) function of the Client.

This probably requires deeper changes to the API of the Client.

[tomaka]

This issue is more or less tied to #91

Doing this change would require changing the Client to have interior mutability.

At the moment, the Client's API accepts &mut self everywhere, meaning that the Wasm node can do things such as adding a chain without running the risk of having to wait to lock a mutex.

The good news is that the JS API is ready to accomodate for an asynchronous add_chain function. Only the internal plumbing would need to be updated.