basic async service

[rpaaron]

#491

naive prevention of handle_request from calling send_response. Instead let the client do it, when the result is available.

[rpaaron]

example client would look something like:

auto service = node.create_service_async<AddTwoInts>(
    "/add_two_ints",
    [&](const std::shared_ptr<rmw_request_id_t> request_header,
        const std::shared_ptr<AddTwoInts::Request> request,
        const std::shared_ptr<AddTwoInts::Response> response)
    {
        std::thread t([=]() {
            /* something that takes a long time */
            response->sum = request->a + request->b;
            service->send_response(request_header, response);
        });
        t.join();
    }
});

[ivanpauno]

I don't think we need more API for this, we can already do it using #1709.
@rpaaron Could you check that PR?

[ivanpauno]

I don't think we need more API for this, we can already do it using #1709.

Unfortunately, I didn't see this PR when working on #1709, but the implementation is pretty similar.

[adityapande-1995]

@rpaaron should we close this PR, now that #1709 has been merged ?