27 rpcresult improvements

[eigen-value]

This PR addresses a thread related concern: concurrent calls to .result

Besides it introduces a change in the API that breaks backwards compatibility, but improves readability:

// no longer
// RpcResult async_res = Bridge.call("metod", param);
// async_res.result(res);

// after this PR
RpcCall async_call = Bridge.call("metod", param);
async_call.result(res);